Jane, have you ever left them with strange cats, or had strange cats over in their space? (Basically, do you know if they're alright with "new" cats?)
I
could be willing to watch them at my place, depending upon how well they might get on with my cat.
Bonny (my cat) has been pretty good at dealing with visitor pets in her space but I know not all cats are so complacent with newcomers, however temporary they may be.