DX12 was supposed to be universal, able to run 2 cards simultaneously, but not in sli/crossfire, so you'd actually see a benefit from smaller cards as the ram would be additive, not shared as mgpu can run either AFR or SFR separately on each card. The problems usually arise when running a DX11 title, which does support sli/crossfire and does not support mgpu. Unfortunately, game devs haven't really utilized to any degree mgpu support, so the advantage of DX12 is kinda moot in that respect.