My girlfriend and I each have an indoor/outdoor cat; what is the best way to get them used to each other (without killing one another)

She has Maggie. Maggie is 10 and is a diva. She’s constantly after critters outside, my dog is scared of her, and now with Frank (my boy), she hisses and spits when they’re inside. I rarely bring Frank over to Nikki’s. This is the second time, and both times, she’s been a kitty bitch ;-) Frank’s so easy going, but, he’s not the type of cat to run and hide. He just sits and they stare at each other. Every time that she sees him, she hisses. He hasn’t hissed once. He actually seems pretty relaxed in that he has no problem sleeping comfortably here and cuddles with Nikki and I. She, on the other hand won’t come near and wants to get the hell outside.

It’s a small apartment and like I said, I rarely have them together, but, yeah, I’m looking for a way to help them get used to one another.