Authenticate to the single SIP trunk locally on the TA908e itself.
Create two voice trunks, one for each PRI.
Create two voice grouped-trunks, one for each customer, and include the respective PRI in each.
Populate each grouped-trunk with the digit patterns for that customer's numbers.
voice trunk T01 type isdn
connect isdn-group 1
voice trunk T02 type isdn
connect isdn-group 2
voice grouped-trunk cust1-tg
voice grouped-trunk cust2-tg
You may need to tweak the number patterns if your SIP carrier sends a leading 1, etc.