I'll be using the In Win A1 Plus, and i could not for the life of me find anything on running two 120 rads in a build.I would love to know if this is a good/bad idea? Also, if i should just mod the case to incorporate a dual 120mm on the bottom? I have absolutely no modification experience.
Well like everything a lot depends on how you use it and set it up.
Thermal conductivity is mostly a constant. It's so many Watts/Km. So if you set up the coolers in series, your entering temperature of radiator 1 might be 92F and exit temperature of 84F. The inlet temperature of radiator 2 would be 84F and exit temperature of 80F.
As you can see the second identical radiator, is less effective (8 degree delta versus 4 degree delta). The difference in air temperature between the liquid entering the second radiator is less than the first radiator. So there is less energy transfer (Newtons law of cooling)
If you set them up in parallel circuits using a T-splitter, the flow rate to each radiator is cut in half. This means more time for the radiator to exchange energy. But Newtons law of cooling kicks in again. So you'll get about the same 80F exit temperature.
Not only this, but if you run the loop in a series to more than one heat source (ie: GPU -> CPU -> Radiator) you're dumping warm coolant into the CPU which affects its performance. In such situations, it's best to have two radiators, and two pumps. One loop for each device.