YES YOU CAN!
ok what I understand is you want to BRI ISDN lines and use only one IP for both physical connections.
try ppp multilink, as isdn support ppp encapsulation create a logical multilink interface and budle together both isdn bri lines, so you are also load balancing by fragmenting the datagrams.
take in mind that you must configure ppp multilink in both sides (mean yours and provider one).
