No expert advice by any means, but leathers leach chemicals into water column as their method of warfare.
Mushrooms have very short tenticals that pack a sting typically not a issue with having corals near them, though it Maybe a issue in your case due to them actually touching each other. In the long term I would assume eventually one would take out the other.
Of biggiest concern to me would be the
chemicals they could leach into the water effecting other corals.
I have a few different leathers, and I typically always run carbon in the filtration as prevention to absorb them. Yours is still pretty small so it may not pose a serious issue.
If it was me I would attempt to separate them from each other. That way they both are happy. Nice mushrooms by the way.