Hello!

How to unwrap multiple objects on one uvmap, so that when I import these two objects in a game engine, they as a group, use one material? Is that even possible.

Example:
I model a tresaure chest that consists of two parts, but is exported as a group. I do this to animate the cover inside unity engine. But I don't want to use two materials.

yes its possible in houdini if you have multiple object just create a group on each object and merge all of the object apply the autouv or uvunwrap then if you export as .obj it will recognized as different object or if you export via alembic you can create an attribute for each object and export with the name of the attribute supplied as the separation…
if you give a file i can show you what i mean better

best regards
Patar