Hello Josh,
You're right, in the case of a virtual two-drive autochanger, if we configure the second drive (index 1) as read only, the drive index 0 (read/write) will be selected for reading and there will no available drive for writing since the second one is read only.
The way this could be solved is configuring the first drive as read only. This worked for me for a two-drive autochanger and virtual full backup.
I've not tested this with physical autochangers.
Best regards,
Ana