DISK and TAPE are 2 separate backups, you can take a DISK backup without affecting TAPE at all or vice versa.
Just allocate only disk channels in run block with format where you want to take the backup and run the backup from there, it shouldn't affect tape backups at all.
You can run separate backups to Disk and Tape. However, the REDUNDANCY policy applies to the combination of backups, not to each target location separately. Therefore, you may find backups being marked obsolete.
Hemant K Chitale
recovery catalog is recommended.If the target database controlfiles are lost recovery can become difficult if not impossible.Having recovery catalog makes the restore/recovery easier at the time of critical scenario. Even for larger system the use of a recovery catalog can increase the backup performance.