This isn't a wedding invitation, so the host's names don't have to be listed. This could be a regular dinner party invitation. Just inform your guests the what, where, and why of it.
If you really want to create a formal invitation and include your names, you could write that Mr. and Mrs. You and Mr. and Mrs. Them would like to invite you ... .
The ex wants to have names for the hosts of the rehearsal dinner. My question therefore would be - who's name goes first, the grooms father and wife, or the groom's mother and husband?