Hello @ian 
Nice to see you again!
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Yes. these two sensors have similar functions. (but its have adv and dis adv) Some users used to camera and others prefer Lidar. So they can use both or one of them what they purpose.
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Of course. You can attach more dynamixel to TB3.
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dynamixel used TTL and RS-485 communication and it supports daisy chain . So you can connect it over twenty motors.
Thanks
Darby