If you created the second container by copying the container file, instead of running 'Create container' operation, and try to mount both containers at the same time,

the system will mount both containers, but won't show the second container in Windows Explorer.

The problem is specified as "signature collision" and it occurs on Windows 7 and later versions of Windows.
Since Windows 7, the system does not allow two drives with the same signature to be accessed simultaneously. The second drive is mounted, but gets status "offline".

To overcome the problem, you should create new container using BC Control Panel, and use it instead of copying container.
Other solution is: after each mounting - open Disk Management utility (click Win+R, type 'diskmgmt.msc'),  right-click on the "offline" drive and run the command "Online":