There is a recent evolutionary trend for flowers to grow in pairs. It even turns up with flower heads containing multiple smaller flowers, as seen here with this New England aster. The only explanation is that pairs of flowers fit so well in the standard frame used in photography. In my photo album, you can see ample evidence of this phenomenon. This adaptation gives species a competitive advantage and helps them survive when others are cut. It also makes them more popular for cultivation.
While I sometimes chafe at the redundancy of the resulting images, it is very hard to resist. Please do not publish references to this discovery. I have not yet been able to get it published in a peer reviewed journal.