Might want to mount one bucket but download from the other (SOURCE_BUCKET vs WORKSPACE_BUCKET).
(e.g. reading images off of the public bucket Cell Painting Gallery, mount is good but workspace files are likely in another bucket)

We currently only support:
Same bucket - Download = True for both
Same bucket - Download = False for both

I think we want to support:
Different bucket - Download = True for both
Different bucket - Download = False for source, Download = True for workspace

I think we don't need to support:
Different bucket - Download = False for both (i.e. mounting two buckets)

I'm indifferent to supporting:
Different bucket - Download = True for source, Download = False for workspace