That should not be a problem.
What you do is for SBC1 you create sip trunk to SP, with a Trunk Account and Trunk Group. Then you would create 3 Trunk Accounts to the SIP PBX, each with one of the IPs of the PBX. Then you would put all three of the Trunk Accounts in a Trunk Group going to PBX.
Trunk Group to PBX would look like this:
SBC2 would have the same configuration.
It would be up to the SP to route the inbound calls to SBC1 or SBC2, When the SBC receives a call it will send it to one IP on PBX, next call would go to the 2nd IP, and next to 3rd IP, then start back at 1.
Let me know if you have any other questions.
Here is a helpful config guide: