I’m running some tests for multi-instance render with Cycles and so far the results are not good. I think my system may be bottle necked since system RAM, cpu, or disk pegged at some points. So my results may not be representative of higher spec machines. Will publish results.
Two cards is nearly 2X as fast as one for single instance using both cards so that is a benefit.
Update:
Results https://blenderartists.org/t/splododynes-splodebook/1292145/11