I highly recommend reading a lot of MG published within the last few years, same for YA. A good, solid understanding of the markets you're attempting to merge would go a long way toward helping your ability to do so. I've seen agents and folks around here recommend reading 100 recent books in any market you want to write in, and I've only recently realized they weren't joking. When I decided I was meant for writing MG, I started reading right away. Now I have about 30 titles under my belt and I still feel like I'm only scratching the surface of understanding the MG market.

Also make sure you get beta readers who are familiar with the markets. It may be hard to get one who is an expert in both MG and YA, so I'd just look for a few YA beta readers and a few MG ones.