I'd also leave them together. The "clumping" habit is just a description against those that have a running habit. Common milkweed for instance will send runners out 10' away from where it's growing. It's the reason I dug mine up this week and moved them to behind the garage and planted a different better behaved milkweed that clumps. So you do want them to stay into clumps, it's their natural growth habit.
If you're trying to stretch them out and get more plants out of them, I suppose you could try separating them, but that would be more stress on both of them when you separate 2 of them. Best way is just plant them as is for best chance of making it.
