When you do the daisy chain the drive should be of same make and model. LTO3 with LTo4 is not recommended and will not work. Instead you can connect both the drive as stand alone and use them with BE 2010
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.