Target YA agents and editors. They are well informed about the adult market and understand crossover potential when they see it. Agents and editors who specialize in fiction for adults tend to be limited in their knowledge of the YA realm and are more likely to see the audiences and marketplaces as separate. It’s okay for you to say that you think the book has crossover potential in your query letter. Stress that the issues are broader than pimples and proms, and that the richness of your ideas has the potential to satisfy all ages.