If you are worried about heating the barrels, you should not be building doubles. You will have to heat them to put the ribs on and to regulate them; remember they will be bent in the regulation process. And to attach the quarter rib. More heat to solder them to the monoblock won't hurt anything.
I would never consider a heat shrunk barrel to be safe in a home built double rifle as the holding strength of thin walled monoblocks is not much. I use threads, with or without solder; doesn't seem to matter. There is not much thrust on those threads so the pitch doesn't matter.
And coarse meaning, not fine, is spelled, coarse.