Problem is the emerald might not move up at all because he's being shit on by grandmasters vs the other emerald having a field day beating on golds lol
As long as the MMRs are right, the high Emerald should hang just fine w/ GMs (50/50, reasonable KDA, etc) and the low Emerald should be Golding like Golds (also 50/50,and normal KDA).
MMR should be a good indicator for both ability to win and ability to play reasonably well with those of the same MMR. It's definitely not perfect, and we have improvements to it coming, but it does appear to be more accurate than Rank.
Emeralds with GM-level MMRs definitely destroy Diamonds with Gold-level MMRs.