I've known several people who have run marathons as a training run. One of the two things happen:
They approached the race as a training run and ran at an even pace, logging a time a few minutes under their goal time. On race day they're all pumped up and go out harder and end up crashing-and-burning, finishing in a time SLOWER than their training run marathon.
OR
They get an overuse injury during their training run and miss their goal marathon.
I haven't heard of a runner using a marathon as a training run then popping a great time in their goal marathon. I'm sure it has happened, but I think it is less likely.
My suggestions:
Don't ruin your goal marathon by running a marathon the month before
Just consider the money spent on the March race a loss
If you discover you are able to recover very quickly from a marathon, you can consider trying this in the future
With all that said, good luck in both of your marathons. Because by the tone of your post, I can tell you're going to run both regardless :o)