The question here is not about a system with two PSUs but about splitting power within a single GPU across two PSUs. 6/8 pin(s) on one PSU and the riser power on the other.

Ideally they would be on the same supply so that fluctuations in power are equal. Whether single rail or multiple rails the ground is common. That said, on the GPU board the components are isolated so the 12V and 5V voltage ripple should not cause any interference. That said, there is the possibility of unexpected things happening if power were to be cut to one psu and not the other while running. Mostly it would just shut the GPU down.

So can you? Yes.

Should you? I would rather not but I like to keep everything a bit organized. It would likely be fine for the short term but if it can be remedied in the long term best to get them on the same PSU.