[harlequin2]

Yes, it can be connected that way and I realised that way back when I started this project. However, I wasn't sure how much interference the comms would suffer if one of the lines was also carrying the motor current with its high level of emi and so I decided the twisted pair would give the best protection. For that reason also I only run the data at 9600 bps which is quite enough to do the job and less prone to interference than higher Baud rates.
I haven't looked at doing away with the opto, but I guess that would be possible too.
I see the one described in the link you included runs at 56 kbps and uses a single wire, so maybe I was being too cautious. I have my doubts though, especially when I see a 2R7, 5 watt resistor being asked to carry 1.5 A of balancing current and thus dissipating 6 watts! In my experience, a 5 watt wire wound resistor can dissipate only about 2 watts in free air before catching fire.