The Priority Pass provides $28 of credit to its holder and his/her guest ($56 total) at select airport restaurants.
If my friend and I both have priority pass, could we get $56 per person by being each other's guests?
The final say would be up to the restaurant themselves, however there is nothing in the rules that would stop you from doing this, and no reason that I can see that the restaurant wouldn't allow it.
Keep in mind that this would only be beneficial if your Priority Pass membership includes a free guest - most don't, in which case you would need to pay a fee to Priority Pass for the guest, which might be a higher price than the benefit you get from doing it. That said, some do (mine does!) so this would be a viable option.
EDIT: It would appear that some locations are starting to crack down on practices like this. One location that I know where this was previously allowed at is now stating that "Only one $28 credit is allowed per person that is present and consuming food or beverages". As stated in the original answer, the restaurant would have the final say, so ask them before presuming anything!