I have a gravity flow all grain system with cast iron burners an a high pressure regulator. I've never had any problems until last weekend. As I was heating my mash water the burner went from roaring to just working. I took the jet out and used an high pressure air to make sure it was clear. Put it back together and it seem to work fine. Then when I was getting ready to sparge my second batch the sparge water burner did the same thing. And any time I have more than one burner going at the same time they barely work. Do you think it could be the regulator??
How many BTU's are you pulling between them and how large of a fuel tank do you have?
If you are pulling too many BTU's from too small a tank, you can ice it up.
Your regulator could be malfunctioning and not allowing the volume of fuels you need for both burners.
The manufacture said the the burners put out 170,000 BTUs. I suspect it's less than that. I have a full 5 gallon tank. I have had all three burners going full blast in the past, just to see if they would work and they worked great.