Rescue vs grand-cat

Hi all. Just recently rescued the neighborhood stray (Winston) and he’s has been comfortably living with us for about 3 weeks and he’s a joy. Very chill. Fine with our dog. He had the crap beat out of him by another stray cat when he was out in the “wild”, so not sure how he handles cats. I also babysit my grand-cat (Monty) a few times a year for weeks at a time. He is very friendly, and the exact opposite of chill. When he was a kitten he played with other cats but hasn’t socialized with them for about a year. My son has a military assignment and just asked us to watch Monty for a month. I am scared to death this is going to be ugly. Winston has a safe space with us and Monty knows our home well. Do I do the whole “separate-in-rooms-through-closed-door process”?? I’ve already bought the calming plug ins and set up the basement where Monty does like to play, but there are no windows and Monty loves being with gramma and grandpa, but Winston is finally safe and super thankful for us saving him and loves to cuddle….i just feel so guilty locking one of them up. Help!