Too much restriction with that T pipe, I'm not surprised you can not get enough heat. If it was me, I'd cut that out of there, make 2 separate fire boxes, one for each tank, you should be able to use the firebox you have now and turn it into 2. Those tanks are large and take a lot to heat up, which means they need a lot of flow. IMHO
Michigan Custom 'Que Team
"Serving Those Who Have Served US"
"Good judgment comes from experience, and a lot of that comes from bad judgment." -- Will Rogers