How do I adapt 2 cats I am temporarily fostering- fixed and age 5ish to my 9 yr old fixed male cat? I have had them seperated by rooms and doors for 2 weeks now. Have used cat carrier and put mine in their shared large room. Need them together.

Your best bet would be to call them.

Have someone hold one and you hold another and let them smell one another. As long as it's been, you should be able to let them out and see how it goes. They will hiss and maybe swipe a little bit but that's perfectly normal while they adapt. It shouldnt last more than a few hours. If they are attacking each other aggressively, that's a sign that they may not get along and will need to be an only cat.