Hi Provence.
While our combined members like yourself have the option to have another combined member stay in your home and you can stay in theirs. Not all of our members have that ability, by only asking for members who can do a house swap you are restricting the number of applicants that can apply and excluding a large number of sitters who may be perfect to look after your home and pets.
TrustedHousesitters is not a house swapping site, and while we did discuss if this is an option we could branch out into we felt it did not align with our ethos and would be unfair to our sitters who could not provide a home they can swap.
You can mention that you are open to having a combined member swap so that you stay in theirs and they stay in yours, however, we do not allow owners to only ask for swaps as this will affect the number of applicants and their success on the site.
I hope that helps.