Connect two meshes by closing the gap between them.

These models (Goethe and Lion) have an open area which we will use to join them together.

Scale and position the two objects so they line up as best as possible. (These positions and plane cuts could be improved)

Open the object browser (view > Show Object Browser) and select both objects.

Group the objects together (Edit > Combine). This doesn't change the meshes, but treats both objects as one.

select the area that includes the two open areas we want to join. Click on the menu 'select' and start making the selection by clicking in the background. Complete the selection by connecting the red line back to the first point. If there are only two open areas, select-all works fine as well.

This is the resulting selection

Join the objects together: Click on the menu 'select' > 'Edit' > 'Join'. Click accept.

This is the result.

The new filled in area interpolated between both open areas.

Go to menu 'Analysis' > 'Inspector' to check for possible errors and fix them.