What would you have to add to a two-season sleeping bag to make it a four-season bag? ie would you get away with a sleeping bag liner, or would you need another two-season bag (ie one inside the other)? Or is the whole idea completely inmpractical?
Not completely impractical, though the use of the term season to rate a sleeping bag is...There is general opinion that it gives no real indication of what conditions a bag is suitable for. On the other hand sleeping bags are extensively tested and have temperature ratings clearly marked on them which allows for much more accurate judgement.
And in answer to your question it is entirely possible to layer up with sleeping bags, especially if you are buying them for that purpose and cant buy them to fit i.e. one bigger bag and one smaller one. But again you lose the clear indications of warmth and will have to experiment a bit. Also would not use down for the internal bag as it is more likely to be squashed and lose its heat retention capabilities.