I for one would not use a 20ga action for a rifle. However pressure is pressure and if the pressure per square inch for the rifle cartridge intended falls below the pressure per square inch of the origin proff loades of the gun then in theory one should be all right using the action.
The gun MUST be reproofed at around 30% higher pressure than the recomended loading for the cartridge!!!! If the action does not have origional proof marks which can be used to determine the origional proff pressure then leave it alone. As for shotgun actions being OK for rifles, only for low pressure cartridges 44K P.S.I. or less, (C.U.P. is not the same) for the most part only the best actions will meet the strength requirements and good doubles are not cheep. There is a guy up here who will build you a double 45/70 on a Savage 311 action. He sleeves the barrels and ends up with a 15 or 16 pound club!!! I also think he is asking for a law suit and needs a shrink. Cheep actions and glued in barrel liners is a good way to get killed or kill some one else. If you want to build a double rifle on a shotgun action but it is not going to be cheep if you want to do it right and be safe!