are you running an NVLink? I have the same setup but no NVLink and it feels like it's best just splitting the 3090s to run separate models concurrently. But I also have no idea what I'm doing.
AFAIUI, there'd be little advantage in having a higher speed inter-card connection, because the cards don't really talk to each other during inference. The loss of efficiency compared to a monolithic memory architecture comes from scheduling, not from data transfer.