RMAN Backups - Can more than one media management software solution be used?
Creating the symlink $ORACLE_HOME/lib/libobk.so pointing to the media management library is not a strict requirement. As part of RMAN channel allocation, the path of vendor speficic library can be specified in the SBT_LIBRARY parameter.
I never tried this, but just by allocating the SBT channels with different parameters should make this work.
UPDATE:
For your other question about restoring backup pieces from different media:
RMAN has a feature called Autolocation. The RMAN SET AUTOLOCATE ON command is deprecated since 10.2 because this feature is enabled by default.  RMAN only needs to track that for example the backup of datafile 1 is included in backuppiece vlv6su4u_1_1 with channel type SBT_TAPE. Using this information and the autolocation feature, RMAN is able to recognize which channel it should use from multiple channels allocated with different options, simply by querying all channels for this backuppiece. This feature was invented for handling backups taken on different RAC nodes, but I was able to use it in another scenario as well: for example after a TSM backup server was migrated (e.g full backup from old TSM server + file space, incremental and archivelog backups from new TSM server + file space, channels allocated with different tdpo.opt files pointing to old and new TSM servers).
The same may work with 2 different MMLs as well.