Of course you can use a DD for multiple backup devices.I do this all the time. I do not remember if each will create its own M-Tree. My personal guess is that there is just one M-Tree per 'DD-Client', means NW server/snode.  
I have a backup policy for VMware virtual machines which performs backup to primary backup storage (Data Domain) and a clone action which clones the data to physical tape library with longer retention. This is running fine, but when trying to restore a VM from a tape I cannot see any progress being made.    The recovery log contains only the following:  155752:nsrvproxy_recover: Unable to find a usable device for recovery of saveset 407484030: Unable to find enabled Data Domain device for volume '462APSL6'. An attempt will be made to perform automatic clone resurrection165713:nsrvproxy_recover: Attempting to automatically resurrect clone 407484030 with 7 days retention to DD volume in pool Data Domain Default Clone  Also in alerts tab I can see the following warning:  Waiting for 1 writable volume to backup clone pool 'Data Domain Clone' disk(s) or tape(s) on networker for client(s) vcenter   I am not sure what is wrong, I assume that it tries to use the Data Domain as a staging area for the restore but something seems off. I am using "Data Domain Default" for primary backup job and a custom "Clone to Tape" media pool for the clone job. Networker is installed on Windows Server 2019 - NetWorker 19.4.0.2.Build.127[IMAGE][IMAGE]
Attempting to restore data from tape clones