No knowledge of the EK, but if I took a stab at it, I'd consider the fact that they're lovers could explain why they co-ruled in the first place as, under normal circumstances, they'd have tried to oust each other. Since they were lovers, though, they ruled together, until things went south. It wouldn't matter if it was socially acceptable, since, as rulers, they could ignore convention, or they could simply rule that this is okay. Not knowing how sexuality is dealt with in the FR, I have no context to determine what the society at large would think, much less the specific Gnomish kingdom.