I'm thinking that for 5
and +1/+1, Yang is on the expensive side. True, it affects every freakin' thing you can conceive on board, from Abominations to Wyrms and every freakin' thing between 'em. But we can do better with things like Shards of Patience (+2/+2, at cost of not attacking in that turn).
I think something cool should happen if you have both of them at once but I have no idea what.
I don't think it's practical enough to warrant it, even if Darkness/Light have some good tricks that ties 'em together. The six-card limit is per un/up parallel, not six upped and six unupped.