It can't technically be done through the system, but it's a good idea. I am getting a second unit ready in my house, and I am going to offer a discount to those who book both if they have a larger group. One way you could do this is advertise it on the listing, and they can reach out to you if they want to take the option pertaining to travel the country. You could them message them some sort of discount or special offer to encourage booking of the other. I am going to have an offer where I give 20% off of the overall cost for booking both, and something like that could work in this case as well. A lot of people don't read the listing, which is a downside, but people who are interested in what you listing has to offer will see it, and they are more likely to be good guests.