Actually...after thinking this through I'll either have to run the other tank off a separate return pump, or get a larger return and consider re-plumbing the current setup to accommodate the 2nd tank.
I went through my "what if" worst case scenarios, and using the closed loop to pump water into the 2nd tank introduces a bit of a risk. If the return on the main display fails and the closed loop keeps running...water will be diverted away from the main tank and into the 60p...which will be sending water to the sump shared by both tanks. It'd only be a matter of time before the tank emptied completely and the sump overflowed.
So scratch that idea,

. I'll try to figure out a way to get this all plumbed together within a reasonable budget...but I may put that part of the build on hold if it requires too much more equipment.