I know it is very difficult to work out a budget on a trip like this but I would say that probably yes, you could get by on £20 a day each. I usually guage £30 a day for myself but I am not very good at being frugal! Being a couple will reduce your expenditure quite a lot as hotels charge per room usually.
In Madagascar camping is not really possible. Hotels will cost you around 30000 Ariary per room although in some places they are cheaper, depends where you are going. Park entries are around 30000 Ariary and upwards (you always have to have a guide). Transport is really cheap on the taxibrousses and likewise, food can be very cheap also.
In South Africa, dorms are on average 90 Rand per person and there are cheap eats everywhere. Transport as you probably know is best by Bazbus or hire car. Check web sites for prices. Aroundabout cars are very reasonable.
As you say, Safaris are going to be your major expense together with crazy activities in Namibia if that is what you intend to do. I can reccommend Skydiving, Dunebuggying, Sandboarding etc in Namibia, awesome experience! I can also reccommend a 'Living Desert Tour' from Swakopmund, I can dig out details if you are interested. Very knowledgable chap takes you round the desert in his incredible 'moon buggy', finding snakes, scorpions etc. He also does a night trip.
In Tanzania, shop around in Arusha for best Safari deals. In Kenya, there is someone on here that will probably advise you of best safari places, when I remember his name I shall let you know!!
Money wise though, best rule of thumb is.. take as much as you possibly can, nothing worse than not being able to do an activity as it is outside your budget, although you could be really tight for a few days to cover it. Better to have too much money than not enough!
Have a fabulous trip, I am realy, really jealous 
Check the forums for all these countries, lots of advise or I can help you with Madagascar if you need it.
[ Edit: Edited on Nov 17, 2008, at 3:03 AM by baluba ]
