The Frogman and the Spy by Irene Preston

Spy vs Spy
Jim Calhoun and his sister Lori are just two Americans in North Abarra exploring their roots. They are definitely not off-duty CIA agents.
Enzo da Silva is the head groundskeeper on Princess Odile’s country estate. He is definitely what he seems to be – the guy who trims the hedge maze and measures oxygen levels in the national forest.
The Princess’s birthday bash is a major celebration every year. As the big day approaches, a series of accidents plague the preparations. It’s almost like someone wants things to go wrong. But it’s not as though two commoners like Jim and Enzo – with absolutely no super powers – can stop a rogue supervillian. And if Jim and Enzo keep showing up at the same crime scenes, it’s not because they can’t keep their eyes off each other.
Definitely not.