I just uploaded a beta-test version of a ready-to-run app which supports both the RTS and DTR pins of the RS232, so you can generate two independently-adjustable PWM signals with one RS232 serial port:
http://www.chiefdelphi.com/media/papers/2702
Note that you can drive up to 4 motor controllers (in parallel) with each signal.
Screenshot of the user interface. I'd be interested in constructive feedback from users.