I'm ever so sorry I can't give any advise... As a child mamma sometimes had two fur-siblings at the same time... but the "new" kitty was alway introduced straight away to the "old" kitty... All kitties mamma had as siblings as a child/teenager were homeless kitties... And grandmamma just didn't have the heart to ever let a homeless kitty go without a home! There were never more than two at the time... and the "new" ones always seemed to adapt with the "old" one... the "old" one was mainly "Puss-Puss", as she came as an adult homeless cat when mamma was 6, and had to be put to sleep the year mamma was 17 (due to cancer)...

I shall wish you luck, in the quest for the two little cuties to be friends! Even if they don't become good buddies, they should pretty quickly learn to live around eachother.

Best wishes, Spartacus and his mamma!