The benefit of using NVLink is more Vram (allowing Daz3D more complex scenes with more characters), but render time is actually increased a little. This can be seen in the game, just compare early party renders vs ch9 renders, day and night. With Iray if you don't need the extra Vram not using NVLink is faster.Especially in the sort of workload DPC has, of massed queues of renders. You might get an advantage (with older implementations, not a 3090) on a single render, but when you have a stack of thousands, and are loosing half of your vram (along with other resources)....It probably would be reasonable to put more than one card to a system, depending on the system cpu/ram/pci bus lanes/etc. (server grade stuff, for instance, or at least high end multi-threaded CPUs; that's how commercial render farms work), but you wouldn't want them in SLI, even if it were implemented. Better to have two renders going and using up all your various resources than to do them consecutively with what ever workload can be done simultaneously, but wasting the non-core resources on the second card.
I think DPC has four 2x3090 rigs.