In my experience lionfish are very peaceful and timid fish, I have never seen one show signs of aggression. In the wild I've seen lions stick together, usually anywhere from 2 to 6 lions under a single rock which seems to suggest you can keep a couple of the same species but to be safe it might be best to avoid this in the same way you do with most reef fish.
Here's a clip of just such a find
I also currently have 2 lions in the same tank, one zebra and one fu-man-chu and they don't show any signs of aggression towards each other at all. I have a tilefish, 2 seahorses, a pipefish, a green spotted mandarin, and an engineer goby in the same tank. I'm also planning on adding a third lion (fuzzy) when I find one. I believe provided the lion can't eat the tank mate they will be fine, more often than not you have to worry about the lion over the tank mates. It's best to train them to eat off a stick or tongs so you can feed them easily and you don't have to worry about other fish out competing them for food.