Soldered Support:
I’d recommend using a power supply that doesn’t introduce noise into the communication. I’m not sure what the problem was with USB, since it should provide the most stable voltage.
For RS485 communication, there’s plenty of information here: http://arduino-info.wikispaces.com/RS485-Modules
This corresponds to our modules: Soldered: Open-Source Electronics, Boards, Sensors & Modules – Soldered Electronics
Regular serial communication using SoftwareSerial (TTL logic) to send bits over 30m wouldn’t work well.
RS485 can reach over a kilometer (we have modules for this). RS232 can go up to about 20 meters.
So SoftwareSerial + RS485 module = what you need. That’s my recommendation. Create two SoftwareSerial objects and you can connect two such modules.
433MHz can work too. We’ve done it with one transmitter and two receivers - the transmitter sends info then waits a second, one receiver picks it up and waits two seconds, meanwhile the transmitter sends the same info which the other receiver now picks up (and waits 2 seconds, but that was fine for us). You can do something similar with two transmitters and one receiver, you just need to synchronize the two transmitters somehow (some kind of clock signal). If both transmit at the same time, the info will jam and the receiver won’t be able to read anything useful, plus depending on how you write the program, it could cause issues. You need to devise a system to avoid transmitting from two places at the same time.
With NRF - it’s bidirectional communication on 2.4GHz and sends info through pipes that keep communication constantly open between two points. You can communicate with multiple devices, check the tutorial below.
A colleague is writing a tutorial for this module in Croatian, but here’s the excellent English version just in case: https://arduino-info.wikispaces.com/Nrf24L01-2.4GHz-HowTo
You should be able to achieve 30m on both (just add wire antennas on 433MHz - there are tutorials online).
Using LAN, i.e. Ethernet communication shouldn’t be difficult since there are libraries that unpack Ethernet information packets, but why bother when the solutions above are cheaper and you don’t need 60m of UTP cable.
WiFi is similar, but a bit more complex software-wise (you can create a mesh network for example).
1Wire has that range, but you’ll need to implement that communication in both programs on both Arduinos that communicate (I’ve never done it, so I’m not well informed about it currently - but there’s probably a library).
AREF on Croduino is DEFAULT, but you can also use INTERNAL aref options, and EXTERNAL (of course, connect the desired voltage to the aref pin).
It’s really strange that it works when you touch the wire and it’s not a connection issue. Trig only sends a signal to the sensor to measure distance (and distance info is returned via Echo, how it works is explained here: https://e-radionica.com/hr/blog/2015/08/19/kkm-ultrazvucni-modul-hc-sr04/#Princip%20rada%20modula)
It’s possible that the IIC library for the LCD uses some Timers or delays that interfere with Trig, which can’t send the signal in time to start measuring / read the measurement using the pulseIn function.
Sometimes two libraries turn out to be incompatible because they weren’t designed to work with each other and so they cause problems. I don’t have concrete advice on this. You won’t find another library (that doesn’t use pulseIn) for the HC-SR04, but for IIC you will - but you’ll have trouble and might end up with it not working anyway. If possible, connect the LCD without the IIC adapter.
Original message (Croatian)
isprike, prošli mail sam zametnuo. Slijede odgovori na sva pitanja:
Svakako preporučam koristiti onda izvor napajanja koji ne stvara smetnje u komunikaciji. Ne znam u čemu bio bio problem s USB-a, jer bi upravo on trebao imati najstabilniji napon 
Ovdje je dosta informacija o RS485 komunikaciji: http://arduino-info.wikispaces.com/RS485-Modules
To bi odgovaralo našim modulima: Soldered: Open-Source Electronics, Boards, Sensors & Modules – Soldered Electronics
Obična serijska komunikacija, dakle da kreirate SoftwareSerial port(a to je onda TTL logika) kojime biste slali jedinice i nule na udaljenost od 30m - mislim da ne bi baš radilo
RS485 može i više od kilometar(za njih imamo module, ne znam jeste uzeli). RS232 može do 20ak metara.
Dakle, SoftwareSerial + RS485 modul = ono što trebate. To je moja preporuka. Napravite dva SoftwareSerial objekta i možete spojiti ovakva dva modula.
433MHz se isto može odraditi. Mi smo radili da imamo jedan odašiljač, a dva prijemnika, pa je odašiljač slao info pa čekao sekundu, jedan od prijemnika bi pokupio info i zašteko se na dvije sekunde, a međuvremenu bi odašiljač poslao opet isti info kojega bi sada primio drugi prijemnik(i opet se zašteko na 2sec, ali nama je to bilo OK). Slično možete i u Vašoj situaciji s dva odašiljača i jednim prijemnikom, samo trebate nekako uskladiti ta dva odašiljača(neka vrsta signala takta - clocka). Ako oba budu slala u isto vrijeme, info će se jammat te prijemnik neće ništa korisno moći pročitati + ovisno kako napišete program, mogao bi Vas zbuggati cijeli proces. Treba samo osmisliti neki sistem da bi se izbjeglo slanje s dva mjesta u isto vrijeme.
Sa NRFom - on je dvosmjerne komunikacije na 2.4GHz te šalje info kroz tzv. pipe-ove koji konstantno drže komunikaciju otvorenu između dvije točke. Možete komunicirati međusobno s više od jednog, pogledajte dolje na tutorijalu.
Mislim da kolega upravo piše tutorijal za ovaj modul na hrvatskom, ali evo za svaki slučaj (odlična) verzija na eng: https://arduino-info.wikispaces.com/Nrf24L01-2.4GHz-HowTo
30m biste trebali bez problema ostvarit na oba(samo na 433MHz dodajte antene u obliku žice - imaju tutorijali po netu).
Preko LAN porta, tj. Ethernet komunikacije ne bi trebalo biti teško jer imaju libraryji koji otpakiravaju Ethernet pakete informacija, ali što ćete se gnjaviti da su Vam rješnja iznad ipak jeftinija - i ne trebate 60m UTP kabela
WiFi slična stvar, ali još malo softwareski složenije(možete napraviti mesh network npr.)
1wire ima taj domet, ali morat ćete implementirati takvu komunikaciju u oba softwarea na oba Arduina koja komuniciraju(to nisam nikada radio.. tako da trenutno nisam baš informiran o istome - ali vjerojatno postoji library).
Aref je na Croduinu DEFAULT, ali možete koristiti i INTERNAL aref-e, kao i EXTERNAL(naravno, spojite željeni napon na aref pin).
Stvarno mi je neobično da proradi kada dotaknete žicu, a nije stvar do konekcije.. Trig samo šalje signal senzoru da izmjeri udaljenost(a info o udaljenosti se vraća putem Echo, princip rada ovdje: https://e-radionica.com/hr/blog/2015/08/19/kkm-ultrazvucni-modul-hc-sr04/#Princip%20rada%20modula )
Moguće je da IIC library za LCD ima neke Timere ili delayove koji onda smetaju Trig-u koji ne može pravovremeno slati signal za početak očitanja / očitavati očitanje u pulseIn funkcijom..
Nekada se zna dogoditi da su dva librarya nekompatibilna jer nisu zamišljena da rade s nekima drugima te im tako prouzrokuju probleme. Nemam konkretan savjet u vezi ovoga. Drugi library(koji ne koristi pulseIn) za HC-SR04 nećete pronaći, za IIC hoćete - ali ćete se namučiti i opet možda na kraju neće raditi. Ako možete, spojite LCD bez IIC adaptera.