Update two database items on same form

I have two tables, Rooms and Guests. I have a screen that displays the room information and then lists the guests in a separate table. I would like to be able to “move” the guests to another room, as selected in a dropdown on the same form.

I have all the GUI pieces, but when I try to add an action to the reassign button, there is no way to add an update to the guests in the room.

Any suggestions would be appreciated. I am on the trial version, 9 days left to see if I can build this app before I decide what plan I need. I would rather stay away from the Team plan if at all possible.