Running on Windows 10 and a recent version of Slicer (shown in picture)
When I add a second .mrb file (I’m aiming to compare two different scans) with File → Add Data, the scene goes blank. This doesn’t appear to be a RAM issue.
A .mrb file has an entire scene inside, so perhaps loading one on top of another could lead to some inconsistency. If you save the data in non-mrb form you should be able to load both volumes into a new scene.
Probably you need to select what you want to show in slice viewers (due to this issue). For example, you can go to Data module and show the two volume overlaid on top of each other using the visibility column: