I'm not an experienced runner but I'm just back from the physio who said it's advisable to take 4-6 weeks to fully recover from a marathon.
It took me the best part of a week to be able to walk after my first (and second) marathons this year but I was pretty ill-prepared for them to be honest.
I certainly wouldn't have wanted to run a marathon a week later.
As others have said, I'm sure it's do-able if you avoid injury, but raising £2,700 is a big ask for one person and probably more difficult being two distinct charities. How do you make sure that you don't get, say £1,500 for the one charity and "only" £1,200 for the charity you need to raise £2,000 for?
I got a vague offer to run VLM 2013 from a charity but only if I could raise the requisite money as per usual - they would be happy to defer to 2014 and I'm sure you're charity would too to allow you to focus on the other marathon / charity in 2013.
Good luck!