Now I'm sure you'll have come across the maxim you are not supposed to cross genres but some of my favourite books do exactly that. Jennifer's series does so brilliantly. The most famous example of cross genre work is J.K. Rowling with her Harry Potter series - boarding school stories meet magical stories.
When done well, crossing genres can create a complete new sub-section of fiction and bring new life to the two genres crossed. Christopher Booker's The Seven Basic Plots feels there are only so many plots and so stories are going to come into at least one of the categories he lists in this book. (A long read but a very interesting one and well worth checking out).
In my own case I cross flash fiction with fantasy, sometimes with crime, sometimes with horror and have a wonderful time doing so! And, of course, there are those books which are hard to categorise but you just know you love them when you read them.
So mix away but choose your ingredients carefully! I think it essential to have a thorough love and knowledge of the two genres you're crossing (so you could work well in either if you ever had to pick one because a publisher or agent wants you to do so. I also think there will be a stronger element of one genre than the other in the overall mix which is where your natural preferences will take you and this could well be a good guide if you have to pick a category for your work to go in). It will also show through in your writing that you know both genres well and, as a result, your story will be so much more convincing to the reader.).