Tube sets are common ....on skeet fields ...where you have a 12 GA tubed for 20ga, 28ga and .410..../ but that means you have a heavier gun in a 20ga - than in the 12ga.
Depends on what you want.....
Personally, I prefer stand alone guns in each gague.....unless you want to spend the money for a " carrier barrel"..../ carrier barrel is bored out to accept the tubes - and it's only shot with tubes in it. You shoot the 12ga barrel as a stand alone gun / then carrier barrel & 20ga tube set as a 20ga - fit to the 12ga receiver. Weight is then identical in all 4 gagues.
Browning makes some models in 2oga, 28ga and .410....typically made on the 20ga receiver...so, in theory, the 20ga, 28ga and .410 are all the same weight --- and I was able to buy one of each gague in citori xs skeet model, with parallel combs in 30" barrels ( each weighs about 7 1/2 lbs.....so I put 8oz under forend and 8oz in stock ...so they swing like my 12 GA xs skeet with 30" barrels...../ add about 10 choke tunes for each ga and I have a nice set of guns....
Had I been a serious competitive skeet shooter from the start....I would have gone with carrier barrel & tube sets early on.
I was a bird hunter....then sporting clays ...then skeet ..../ I added a stand alone 20ga pretty early ....could still have gone to carrier barrel, etc. ...but kind of liked having all 4 stand alone guns..... train grandkids, more guns ( mostly just having more guns)...
In my opinion, tubing one gun ....weight difference will drive you nuts....
Carrier barrel is a great idea....great for competition.... ( all 4 gagues are the same )...
Having a few more guns...is always fun....
Or buy a 20ga...in a semi auto...