Ok, option two is basically,
building a loop for each Pc, then decide if you are going for a huge rad to cool both systems or one rad per Rig,
both loops will exit the Pc and go to the other Pc, (via the rad if you go that route)
The pumps will have extensions to the loop's Psu to power them and you need to make a on/off switching lead to enable you to turn the Psu on and off as desired,
when Pc1 or Pc2 is on, the water will still be passing through the entire loop in both Pc's
option three is the same but each rig's loop can be powered on individually,
this way is slightly simpler.
You build the loop for each Pc and have the tubing exit them into a large shared reservoir, this allows greater control over the waters temperature, and noise levels
(cooler water means less noise from fans)
You still need some radiators but the fans can be run slower and quieter due to the increased mass, a variation on this is to make the reservoir out of a Chillerbox, this will cool the water below ambient temperature for much better results
Both ideas are relatively straightforward expansions on a 'normal' loop, it just takes some thought and, planning and some extension making ability
Moto