Just did a test on a sphere with 15K faces, rendered in Twilight on High with a porcelain template applied.
Rendered by it's self with one sketchup open -- 48 sec.
Same sphere copied to a second sketchup and both rendering simultaneously - both finished in 1 min 20 seconds.
Twilight showed both sessions using all 8 cores in my machine. Whether rendering one scene or both, cpu was maxed out.
Not sure, but on a lower spec'd machine, perhaps turning down the priority setting when trying to render two sessions simultaneously could help prevent a bottleneck in the ram.
(My specs are i7 960 CPU, 12 GB of ram, Nvidia GTX 560ti GPU, running on an Asus Rampage III extreme mobo.)