Thank you for the reply!
The robot arms are coordinated (because they use the Coordinated Package, and the deviation from the target curve is not that much).
The speed of the tool relative to the workpiece (during in the middle of the motion, not the start/end acceleration/deceleration parts) is not constant.
I've tried to do the same workpiece and can achieve constant speed (with even higher commanded speed). That's why I'm curious why can't I achieve constant speed in the two-arm case.
The attached file is the speed-error plot. The green line is the speed plot during the motion.
The acceleration/deceleration parts were chopped off for better visualization