@Paul1699:
The only way I know to do this is to have your guest make an inquiry at your second listing and then send her a return offer to accept. You may need to alter her reservation for the first listing before she can book the second listing during the same days but I'm not sure about that. If she is able to book two listings at the same time, after she has accepted the second offer, then do an alteration request to shorten the days at your first listing.
You can do an alteration request and on the screen that has your listing title and picture you can click the title and change the listing, but that will change the listing for the entire reservation. Changing the dates will change her entire reservation dates. Therefore, my first paragraph seems the only way to do what you want to do.