Hi there!
I have a question about merging and voxel remeshing two objetcts together.:
Let’s say I already have a detailed head and now I want to connect it to a newly modeled body. The body in this case has much less polygons, because no details have been created there yet and it is Low Poly.
How can I now connect the two objects without losing (too much) detail?
Simple Merge is the only other way, which will connect multiple mesh’s together, there will be a seem however as they won’t be topologically fused. You can hide it with the clay tool afterward if need be.
You can simple merge as John suggested. I would wait until you’ve done more detail on the body part of the model, building up resolution until both the head and body can be voxel merged with a similar value to retain detail.
The question is do you actually need to voxel merge? If the model has clothes then the join between head and body can be hidden behind the clothes models.
I consider to Print some of my my models afterwards so I thought it is important to merge them. I didn’t know that a simple merge would be enough. So I only have to make sure, that the two parts are connected at some point?
Yes , I’d suggest that you work on the details of the body until it’s finalised and then think about the remesh 
Simple merge for print is no solution, I guess. You need watertight meshes. There should be tutorials out there, but don’t have a link.
I don’t agree. If you know that parts are intersecting that is good enough(for resin) . Watertightness is achieved by closing holes option. I’ve done 5 models which were only simple merged and everything went fine (about 15 prints each time was succesful). Slicer might have problems with model prepered like that but dont worry about that.