My 2 girls have lived together for just about 11 years now, and they just tolerate each other. Actually, I think Celia would be happy to have a buddy, but Margaux is a bully and would be perfectly happy being the only cat.
Curiously, Margaux's more friendly towards Celia when I'm not there. Maybe an insecurity thing, like your Mitsey. When I was away for 4 months, the student who was catsitting/apartment sitting for me took a pic of the two of them lying in bed together, with Margaux grooming Celia. I have never seen the two of them sleep together, and I wouldn't have believed it if she hadn't sent the picture. I'd love to know how they get along when I go away for shorter periods.