See multiport example. This would give you insight to use the two ports for each daisy-chained DYNAMIXEL.
I highly recommend using the DYNAMIXEL SDK as this is more common and familiar to users in the community, so that people would give you hands with no hesitance.
Alternatively, if you are concerned about voltage drop (and current drawing) in series, why you don’t jump the power to DYNAMIXEL? and just use the one common data line in series.
