Of course it can be done. The question is, should it be done, how much would it cost, and is a late model E30 M42 the right place to execute the idea.
I would say no. I don't know where you live, but where I live the smog guy would take a dim view of dual webers on a 1991 car. Not to mention the fuel injection system works exceedingly well, and is very tunable.
I'd do dual webers on an M10 2002 though. That'd be awsome.