Not doing it for rental but we are also looking for beds for when our daughters and family come to visit. We have decided to use our 3rd bedroom (downstairs) as a dining room instead as it would be a waste keeping a bedroom just for when visitors come. We have a king size in the master bedroom (for us) and will put a double blow up bed (not just a blow up mattress but a very good fabric covered one which you can get nowadays and are quite comfy) in the "dining" room when we get visitors.
Upstairs is the problem. We have looked around in Spain and found those beds (called Captains beds in UK) which slide out. As Semijubilada mentions, they look quite odd when pulled out because of the different heights. They do have the advantage of having room at the bottom for storage drawers which is always a plus in Spanish houses. A really good one in UK has the bed that goes under the main single bed with legs that pull out so they are the same size when put next to each other. There is also a divan bed with one that pulls out which makes it more suitable for adults.
The down side in Spain is that these captains beds are quite expensive compared to UK. A single, pine frame with headboard, pull out centre bed with pine drawers underneath can be had for about 200 pounds in UK (not including mattresses). In Spain ,the cheapest we've found, with a metal frame, is just over 500 euros (mattresses not included).
The other option is to get a bed settee but I have found these to be quite uncomfortable as they don't use real mattresses. OK for a night or two but would be a nuisance if someone is there for two weeks.
I think you would be better off having two single beds in one room and a double in the other. Maybe a fold away put you up bed if someone has a younger kiddie staying.