Nick is right here you should have a ball valve or gang valve to regulate flow if your going to run both of them. Unless your pond is very oddly shaped I would say no you don't need both fusers running in that size pond. The 12 foot one should be fine. Move the other one to about 3 feet and use it in the winter to keep an open hole for air to help your fish get the D.O. they need in the winter. You could still use gang valve to swap over or you could just dissconnect one hose and put the 3 foot hose on in the winter time.
Good Luck,
RC
