The Cross-Dressed Union

Both FLs pretend to be a man, in a non-cute more believable way.
In Dominion and Devotion the FL is the emperor, it's more dramatic.
In The Cross-Dressed Union the FL is a scholar, and HIS WIFE is a cross-dressed man, it is more light-hearted.

Both dramas have a FL pretending to be a man. They're scholars with calm personality. They then get married and treat their wives in a very gentlemanly way.
In The Letter from the Cloud, both the wife and the husband are the FLs, and they have their MLs as well.
In The Cross-Dressed Union the wife is the ML who is also cross-dressing.

Both leads live as their opposite gender, get married, then have to deal with the consequences. ML cross dresses for the same reason in both, last son of a military family, while FL is emperor in one, a scholar in the other. Both HE. Leads are much more passable in Union, you need to rely suspension of disbelief for Royal Lie.

Both show reversed gender roles, though for Ye Cheng, the MCs live in a matriarchal society, while in The Cross-Dressed Union, the MCs are both cross dressing and so take on opposite gender roles. Both feature the man marrying into the woman's household as part of a political scheme.

Both leads crossdress and marry each other in disguise. In SL, FL doesn't crossdress for long while in YLYXG, it seems that they're both crossdressing consistently.