All a sleeping bag can do is stop you from losing the heat that your body produces and it does that by insulating you from the cold.
Personally I favour a modular approach, much the same as the layering system used in clothing.
Although this may well be a slightly heavier solution, it may well prove cheaper in the long run, will almost certainly be more flexible and has some practical advantages.
I currently use two sleeping bags, a Down Alpkit Skyehigh 800 and a Synthetic Nanok Endurance -10 with a breathable bivi bag as a shell. Both of these bags are rated down to -10°c which is normally considered as just a 3 season bag in the UK.
One problem that often occurs when doubling bags up is that the outer bag compresses the inner bags insulation reducing it’s efficiency. In this case because the Nanok bag is designed for the military, that often have to sleep in their kit, it is larger than the Alpkit bag which allows the down filling to loft properly.