Ready Steady Eddy

[JAZZCNC]

Ok Both motors drive upto first switch then both back off then Slaved motor drives upto it's own switch while Master stays still then backs off.
So yes if your switches are far away from each other then you can affectively rack the gantry square but that would be silly wouldn't it and no one with half a brain would do that to any great degree. In practice then your switches are so close together they do affectively trip at the same time but that doesn't matter because it means your square anyway. Think of it like Homing the The Z axis then without moving it pushing home again, the axis still moves onto then off switch then goes back to same place if switches are accurate, same principle with slaved switch being at exactly same place as master.
This is why EACH SCREW on the shared AXIS needs it's own switch on it's OWN INPUT so that when they trip together only the Master switch is being watched and when it does trip both motors back off, then ONLY the slave motor drives back onto it's OWN switch then backs off. So if this switch is located at the same place as Master switch (which it should be if everything is square and correct) it drives on and off the switch then ends at exactly the same place.
In practice the amount of movement of slaved motor is so minimal you don't actually see it happen because the trip point is fractions of millimeters away.!!

Yes again to some degree your relying switch accuracy but like as been shown even low quality switches give micron level repeatabilty.

Now with your single switch setup then ever time you home only the one screw is located at the position it started. Lets look at it like this.?

Both screws start at Zero(home) and travel in sync until you hit the E-stop at which point one or both axis lose position. So you then return to home(zero) to re-register Zero position which is defined by the single switch. But this switch is only moving the Master axis back to Zero the other remains at some arbitory position depending on how many steps it lost. This can then accumulate over time so between E-stops and general positional loss thru dropped steps(which will be low if correctly tuned) then you slowly rack the machine and can never truely be sure or certain the gantry is square.!

I've said this many times but if I was using Slaved motors without out Dual home switches then your better off with Hard Stops which locates the gantry square and slowly drive upto to them. Again this way you know you are square and can Zero with confidence knowing both motors are back where they started from.
Other ways is to use cheap dial indicator on each axis and manually turn each axis until reads zero but then you may has well use 2 switches as it just the same.?