Hi Albert,
Not sure what you would want to do here...some ideas:
1. Configure smaller jobs and send them to the 2 LTO3 drives, before duplicating both jobs immediately to the LTO4 tape. This will lead to smaller backup times if you split a large job (for example), and then consolidate those 2 jobs onto 1 single tape for offsite storage.
2. Configure smaller jobs to run to the LTO3 tapes during the week/month, and then use the LTO4 as a month-end tape.
I wouldn't stream jobs to 1 LTO3 drive and the LTO4 drive. It could lead to issues if the LTO4 drive fails for example, and you need to restore from it immediately.