Thanks so much for helping me with this.
Here's where I'm at.
Like you said I made a test toolpath that jogs the Y axis up and down 100 times.
First I ran it with motor A polarity reversed and sharing motor Ys signal from the control board. it worked fine.
Next I ran it with motor A slaved off motor Y with motor A step low activated and it lost steps.
Then I ran as above with motor Y step low activated and it lost slightly more steps.