S.H.
You put them to nap all at the same time.... as you do for your kids.
It does not have to be in the same room.
Keep your eldest, in her own room, as always. Don't change "her" routine. Keep her room to herself.
I assume, you have a place for the visiting child?
I have babysat... and I put my and the other child to nap ALL at the same time. I kept MY kids routines, the same and where/how they slept.
The child I babysat... slept in the living room, in which I had a crib. Just for that child.
It was fine.
all the best,
Susan