I've been thinking of running dual "loops" in my rig. But I don't understand why you you want to run both pumps in the same system. Why not just run 2 systems in one rig, one for some components and one for the other. You have 2 pumps and there's no reason why you can't run 2 systems off of the same res. This is what I've been considering, but only a t-line setup, no res. You'd run 2 comlepetely different systems, in t-line with no shared res so no shared temp exchange, and then the high heat items like cpu and gpu won't bring heat into your lower temp system. In your case this would also solve the 2 different rad metal problem.
BTW- I just ordered a small racing heat exchanger to try out. I've been racing for quite a few years and I can't imagine a heater core could cool as well as what I've seen some of the racing rads preform. After all they are designed to take some severe heat. Plus they are built better and they already have the NPT bungs welded into them so you can run any size/type fitting you want. Heck you could even use some cool looking braided stainless hose with aluminum AN fittings, would look very cool
