From my experience, in principle it is not allowed, however with exemption regarding flights which are operated by business partners of AB, like EY. I had reward flight last year from SYD to BER via AUH, booked as one flight, where SYD-AUH was on EY and AUH-BER on AB. I wanted to book also TYO-HKG-KUL, but it was not possible, because it involved CX and MH and such combination was not allowed.