Have either of you read The City and the City by China Mieville? That's a pretty good crossover novel, but this time merging a science fiction setting with a crime thriller. I've seen this on the crime shelves in my local library and I think that it could open a few eyes to a new genre if an unsuspecting crime lover picks it up. For me it's an interesting mix - you have the fully created world of a scifi novel and all the complexities of a twisting crime plot. I have a Jeff vanDerMeer one called Finch from the library at the moment and that looks to be a similar crime/fantasy crossover.
I'd agree that it's harder to write than a very genre specific novel as you have two very different, and probably equally hard to please, audiences that your book could appeal to. However, in the same vein, this potentially opens up a good chunk of new readership for an author too. If you can get a couple of crime fans thinking that hey, this scifi lark isn't bad, then maybe they'll be willing to go out and try something they normally wouldn't have given a second glance.