Each tape is reserved for a specific stream. If you only have two tape drives, I would recommend setting the combine streams value to 2.
I know the documentation reads that way, however I don't think it works (entirely) like that in practice.
For example, I have a storage policy that is used for SQL database clients. This storage policy has 100 streams, and performs backups of approximately 500 subclients over around 80 clients. All streams get used (new jobs wait for a stream).
The primary copy goes to two media agents locally mounted SAN storage. Due to the DB2/SQL copy quirks with multiplexing - ie. restore is not simple from a multiplexed copy - we do not combine streams or multiplex on the secondary copy.
The secondary copy goes to a shared ADIC Scalar i500 library with six tape heads. The library holds 110 media. We most definately do not go through 100 media per full cycle. Rather, the actual operation of this configuration appears to be that the auxiliary copy will use as many tape devices as possible, and write a single stream to each. When a stream completes, a new stream seems to continue on the same media. If the system reserves one physical media per stream, you would expect that we would require 100 tapes, just for this storage policy. As we've only used around 200 tapes in the past twelve months, this just isn't the case..