You might want to check out Gas forges - Blacksmith forges and knifemaker forges guaranteed to forge weld - Chileforge.com
He is out of Wimberly, Texas. He was at the Balcones Forge meeting last Saturday and had his single burner forge. He also sells a couple of different two burner forges, all of which can be found on his website. He said that they will reach welding temps without a problem. I personally have zero experience with propane.
Hope this helps,
Jerry