Butterfly Bush + one.

My butterfly bush that is growing in the patio is now 3 feet tall and around 2 feet wide. It has lots of pretty purple flowers and is growing so fast. And what was next to it growing?......another butterfly bush. It's still really little but it's definitely another one. I'm kinda confused how they can grow in such a stony area but they do and they're thriving. Both of them will be transplanted this fall