Two 20 mile races is a lot! You could use PART of both to practise marathon pace under race conditions but I'd be very cautious. A lot will depend on what sort of mileage you're doing and how easy you're finding the 20 milers. I'm doing Finchley, 5 weeks out from London, and my plan is 5 miles progressive warm-up followed by 15 @ MP, which will be my hardest long run. In previous campaigns I've run a maximum of 12 miles @ target pace and this has been a real confidence boost without leaving me shattered.
Run both at anything like MP and your training will be massively compromised. I'd consider just doing the first one as a progressive run, maybe use it as practise for fuelling, race kit, etc., and do the second one harder, but keep target pace down to 10 - 12 miles.