I have 1 PCI-X Dual Channel Ultra320 SCSI Adapter card for my p570 server. I would like to split the 2 channels for the usage of 2 tape drives in different LPARs. Is that possible?

For example, I have 2 LPARs running in p570 server, LPAR-A configured the Ultra320 card as pci7 in slot distribution. I can lsslot -c slot and can see the card. The problem is that the 2 channels of this card will be configured together to the LPAR-A.

Is there any way to configure only 1 channel to this LPAR and swift the other channel to another LPAR which in this case LPAR-B?

Tq.

No.
LPAR resources are assigned at the PCI level so both ports of the adapter will belong to which ever LPAR has the slot.

Explore VIO if possible. You may need to buy a license.

If tape drive is only used once in a while, you can always DLPAR the Ultra320 from one lpar to another.

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I understand that I can DLPAR the card to 2nd LPAR for the usage, but I would like to use the tape drive together to shortened the backup window. That's why I would like the 2 LPARs to use the SCSI card (w/ 2 channels) together.
If I made the SCSI as VIO device, am I able to split them into 2? How?

Vscsi sadly only supports disk and optical devices (CD / DVD) at the moment, i.e. tapes are currently not supported.
You could be brave and script the dlpar operation, then schedule a backup on one LPAR, then give it time to finish, scripted DLPAR the adapter to the other LPAR and then back that up. Use cron or similar to run the backup in the window that the LPAR has the adapter for.