If they're the same mount for both arms, shouldn't you just be able to take the center (or bottom) edge of the vesa mount and measure the distance to the edge of the monitor, then if the mounts are in the center of the monitor, double that value, add a small amount for gap then set the second mount at that height?
If the vesa mount isn't quite center, you'd do the same thing, but figure out if you needed to add the upper half or bottom half depending on which monitor you're measuring from.
Kind of like this (assuming I understand what you're trying to do);
If you're planning on spinning them without moving the arms up or down you're going to need a much bigger gap as the monitor's will be at their widest when spinning them at the diagonal. With rotation you'll need to measure from the center of the vesa mount to the furthest diagonal edge of the monitor. As you'll only be spinning one at a time you should be good to measure just one and not add the two monitors diagonal value together.
