Shouldn't be any issue's with doing both. your training plan always assumes your starting point isn't marathon fit so will build you up. If your already at that level after April you just need to keep up the training after take some time to recover from the first marathon.
Depending on your own level's I would think about doing a at least 1 long run each week with a shorter run and also a session on speedwork from the top of my head. over the summer and then depending on how you evaluate your own running in the month or two running up to York increase your miles. If you see any half marathons or 10k's you like the look off inbetween the two races might be worth doing for training, If your up north worth taking a look at some of the fell and off raod races as well.
If you go to the shades thread on RW your probably going to get some better and expert advice through.
If it's of any help putting it in perspective there are a few runner's who will do a marathon every month or so and some who will do one every week but they are quite hardcore.