The grbl firmware actually allows this. That was a big part of the 1.1h update. It reuses the Z pins since Z homes first. You just need an additional hall effect sensor on the left side. Need to update your firmware settings which requires a flash (not to complicated, I documented in this post if you need it).
This should be independent of whatever gcode server you use (CM, CNCjs, gSender, etc) since homing is a function of the grbl board not the software.
Need to see if Carbide will sell just one sensor and mount kit 
@Nazarethii of you’re confident the belts are good, measure across the diagonals of your machine to see if it’s square. Even if you implement a second homing switch, your still tracking on a parallelogram if the machine is not square. The problem your describing is almost certainly a mechanical issue that is correctable, just need to hone in on where it’s starting.