What I did:
- Added cube
- Added multi-res modifier
- Selected simple, subdivided a few times (4)
- Sculpted some details
- Duplicated object
- Sculpted details on new object
When I tried to add both cubes together (as I've done in 2.79) I went into object mode reduced the resolution for to 0 for all options (rendered, scultped, etc.) then added a Boolean modifier and chose "Union".
made sure I was in Object mode and solid surface, not Wireframe.
With further attempts, I found if I select to hide the effects of the multi-res completely (Use Modifier during render & Display modifier in viewport) it works but crashes as soon as I re-enable them.