There's no standard spelling for
Ana Marija/
Anamarija, both are used equally.
Since we don't use middle names, when someone has two names it's assumed to be a double first name and both are always said. In this case the stress is different depending on the spelling and I guess it just depends on what parents prefer (oh, and Ana-Marija is also used).
Ana Marija and Ana-Marija is actually the only standard double first name here. It's comparable to the Spanish
Maria del
Carmen, etc.
Other compound names are just spelled together, like
Marijana for example.