There isn’t an automatic way of combining two instruments (apart from condensing, which is more a graphical representation of combining than what you’re after) but you might find the commands for moving notes/items to staves above/below a relatively efficient way of bumping the 2nd Horn 1 player’s music up onto the 1st player’s staff, leaving you with an empty Horn 1 (2) player you can then delete.
If any of the instruments came through wrong, you can change them to the right instrument.
Additionally, as @pianoleo often recommends, there might well be alternative MusicXML import settings that would produce a better result initially - in Preferences > MusicXML Import.