At the moment I have only 1 c1 so i don't need to think about this too much, but they do make these,

http://www.syscooling.com/products/water_cooling_kits_for_Avalon_II_BTC_mine/2014/1211/635.html

you can compare it to the standard supply

http://www.syscooling.com/products/water_cooling_kits_for_Avalon_II_BTC_mine/2014/1212/637.html

In the end it's all about you local conditions on whether a single rig setup can handle 2 C1's if they are in a nice cold room and you rad's are getting nice cold air going through them possibly. Otherwise you will just run you C1's too hot and shorten their life.

You can also see that the pump in the multi rig kit is a by the looks of it 750 lph compared to 450 lph and over twice the head. Things are never as easy as it seems.

Cheers