You do get two peices of luggage each, even if on an infant fare (no seat for bub), BUT it is two peices only, and like you say if they are light, then you miss out on quite a bit of weight allowance.
You can get buggy bags, Phil & Teds make one as that what we used, think you can get it in UK? But once buggy is in it, you have no chance of getting a travel cot in there too.
However the travel cot is an easier option for trying to put into a larger holdall, then pack clothes etc round it.
If you tried to put them both in the same bag, you would be over the max bag size dimensions I reckon?
Under this piece system, each piece of baggage should have total dimensions (length + width + height) no greater than 158cm (62").
Good luck..... our bub still has jetlag