I can't think for a single moment why pilots (in this case BALPA) would wish to bring in Aircraft Engineers. BALPA is a Professional Trade Association for Pilots (and Flight Engineers although precious few of them left now). As such we have no trade in common, and therefore no reason to join forces.
I get the feeling that you feel under represented by your present lot and seek to improve you position by hanging on the coat tails of any group that you perceive would help you. The answer is not to join forces with BALPA or BASSA or CC89 but to find a way to become united yourselves and get better representation.