The Gods Only Know by Rory L. Scott

Poseidon and Athena. Finally friends.
When the Greek and Roman gods are forced to merge into one pantheon, the unthinkable happens. House Poseidon and House Athena agree to set aside their centuries-long feud in the interest of self-preservation, betrothing their heirs. For close to twenty years, Lukas Poseidon and Daphne Athena knew they were destined to get married, somehow finding a friendship in the face of it all.
Until a year ago, when Daphne disappears without explanation. Or so Lukas thinks. As the threat against the gods escalates, Daphne and Lukas are forced to work together to keep their court happy and themselves alive. While war looms, the tension will either remind them why their families hated each other for years or make them closer than ever before.