No harder than performing brain surgery. And probably about as successful. First, you would need a fairly large triangular rib to solder the two main tubes (where the springs are) together. These would have to be precisely aligned or else it would be a bear to cock since one barrel would start ahead of the other, not to mention the combined spring weights. Then you've got the problem that the barrels are narrower than the tubes and each may have a different amount of droop.
But, let's say you decide to go for it. You get the tubes joined up ok, now you need to get the barrels somewhat regulated. In a good double they should shoot to about the same point. Since they are bound to be out of alignment with each other, you need a method to bring them back to point. CVA tried, unsuccessfully, to produce a good side by side muzzle loading rifle. Never could get those barrels regulated. Their last attempt was to join them at the muzzle with a sort of figure eight collar that was screw-adjustable for spread. The poor shooter had to work up a load and then fiddle with the adjustment until he had both barrels printing at a fixed yardage. Kind of a retirement project, if you know what I mean. You could make up a similar rig that moves each barrel. At least, with the Gamo, you don't have to worry about trying different combinations of powder, patch and ball size, since the shells are fairly uniform.
Once the barrels are regulated you will need to build a custom stock to hold everything. Oh, and the triggers will be too widely spaced to be usable in combination. You will have to make a double trigger (easiest approach) with a linkage to one of the stock mechanisms.
In the end, you will have a very heavy, cumbersome double capable of taking small birds out to maybe twenty yards and throwing your back out each time you mount and swing it. Then again, no one will have anything like it.
ps - "Double-set triggers" are not the same as shotguns with double triggers. In a double-set, one trigger kicks up an internal, spring loaded bar that greatly reduces sear pressure, making the second a hair trigger. In a shotgun, they are simply two individual triggers. I was going to mention that the Viper trigger is not the best design for a shotgun, as I'm fairly sure it's just a regular two stage rifle trigger. Shotgun triggers are made to snap, not pull like a rifle's. The Viper design adds a bit of lock time making snap shooting more difficult. Of course, with a sixteen pound .22 shotgun, that's pretty much a moot point.
