Leo & Cuddles

I adopted a 2.5 year old male cat named Leo from a shelter, where he lived since he was a kitten and was considered a shy cat. Leo's new start in my home was not easy, it took a month to get him out from under the couch, but he is now out and about and very happy, after isolating him in one room and spending quality time with him. So my next big idea was to get Leo some company, for when I am not home. So, I went back to the shelter and adopted another male cat named Cuddles who is around the same age (around 2 years old, just 6 months younger than Leo), and from what I was told gets along with all cats. When I brought Cuddles home I introduced him to one room, where Leo visits and hangs out in. It's now been a week and Cuddles is hissing less when Leo wants to get in his face and they have actually kissed a few times with my encouragement (I play with both of them at the same time with a toy they both like and once they are close together I pet them both and talk to them nicely then ask them to kiss and they do-very cool!) But now, the next challenge is to take the party out to the rest of the apartment. Leo has managed to control Cuddles and goes to attack him if he tries to leave the room. Cuddles has made it out a few times, he is very interested to see the rest, but that's when it becomes a war zone. How do I get Leo on board with letting Cuddles out of this one room without him wanting to go after Cuddles, which then causes Cuddles to hiss, growl, and gets both of them ready to raise their claws? From what I can see, Leo is happy to dominate Cuddles (eats his food, drinks his water and guards the door) with Cuddles being okay with that until he wants to leave his room to explore the other rooms. What should I try next? Advice anyone? It would be greatly appreciated!