I have one of the Hema apps. If I want the other one, do I have to pay?

Yes, you do.

The apps are a completely separate entity. They are different purchases within the app store.

Buying one Hema app does not entitle you to other apps at no cost.

This also applies to different map formats.

You cannot use maps from within the Hema 4wd Maps app within the Hema Explorer app or vice-versa.