Yesterday I was able to shout both my dragons within 30 seconds of each other (should be 300 seconds) I also summoned twin Wraithmen, and Serana conjured two dead vampires to help in the fight. I had a nice little army fighting on my behalf.
I'm not sure if I discovered a bug here, and I have since used the shouts again in a different area's, and again I was able to shout the Soul Cairn Dragon and the Dragonreach Dragon within 30 seconds of eachother, and it works everytime I shout both of them.
Watching two Dragons fight for me is really cool. I'm not compaining about the bug, just wondering if anyone else is experiencing the same??

By the way, the two dragons both act normal. They both land and fight, and they use their abilities no problem.

i have 2 characters with that shout and have no problems with it