You are right in keeping it simple. With .22 lr as the cartridge of choice, that is an easy thing to do! No worries about barrels time, because there is no recoil, no need to worry about pressures, there is none to speak of.

I have seen several double rifles made in 22 lr, but I have never seen one made on a mono-block, most were made from scratch, with the actions actually made for .22 lr as well.

Because of the low pressure, and no heavy regulation drawbacks, anyone with a little "handy gene" in their blood, a file, a propane torch, and vice, can make a set of barrels for a 22 lr double rifle! Just about any way you do it, will work, the only thing that makes a difference, to me, is the quality of the finished product. No matter the size of the chamber, the rifle should be a thing of beauty, and ballance, when finished,otherwise why bother?