It makes it a bit easier and maybe more reliable. One is output the other is input, in the server application I have separate thread for sensors, which run every minute, and a separate for outputs which run every second and execute scheduled actions and any input from the clients. DS18B20s are quite slow, it takes 750ms for one to calculate the temp and I have a few, at some point the software would have a problem with that, I had no proper locking. Even now when it's quite polished I would risk an occasional timeout maybe... But the software should be able to handle that at its current stage, so you are right, I could run the thing on one Mega only.
I also plan to upgrade 18B20s to 2438 which is a far superior chip - it does temp calculation in 10ms and can double as a light or movement sensor.