I presume are talking about having a servo on each end of the gantry ?
Is that a rapid or slow move that it faults on at 20" ? Try a slow jog (30 -60 ipm) to see if it can move down the table without faulting. That will tell you if its a servo fighting each other problem as they move along.
You may have to soften the tune if they are fighting.
Are the 'a' (amps) parameters set the same for both drives ?
Larry