rainsoft ec4 setup

I just moved and it looks like the local service tech can't make it out to my house until August. Needless to say I dont want to wait that long. So I did the install myself. Everything was really easy honestly only issue I'm having now is foolishly I don't know which tank my main in line goes too. I have a 2 tank setup 1 tank has the digital board the other just the line connections and of course the salt bin. So can anyone tell me what tank I connect my main in line to and which tank line goes out to my house.