[RomanLini]

Bearings of that size are normally specified with 300-500 pounds static load radial rating and e.g. SKF recommends not to exceed 50% of that as axial load but there are 2 bearings in the motor. That would leave me with several hundred pounds of permissible axial load and I believe I am not going to exceed that with my 1/4" shaft end mills.
...

That sounded a little extravagant to me, I quickly checked one size34 stepper motor specs in a Pacific Scientific motors PDF;
"max axial load 135kg, max radial load 49kg (radial load measured half way along the output shaft)".

I'm sure I have seen motors specced for MUCH less axial load, I think some motors have a springy wave style washer removing axial play, that was more common on older motors. You should check your motors datasheet!

... (regarding joining 2 motors via timing pulleys)...
But that worst case scenario the active motor (when moving slow) may still try to push one gantry leg forward with 400 pound force (assuming 50% nominal current, 800 in-oz effective torque with the 15mm pitch screws at 80% efficiency). I believe I am building a pretty strong machine but I am afraid it may be a bit too much.

You should be fine with your well thought out design and overkill (ie the big motors and safety timing belt).

It's not really likely for one motor so skip steps, as the minimum distance it can skip is a full pole cycle which is 4 full steps or 1/50th of a rotation. That should not be possible as the timing belt won't allow 1/50th of a rotation difference between the motors. They should share the load quite well and back each other up long before any one motor can fail. It's more likely the 2 big motors will rip your wooden machine apart before they fail!

Regarding the torsion flex Ger21 brought up, if you used 2 thicknesses on the top and bottom gantry plates that will increase its torsional strength in a big way for very little cost in materials or weight, and without needing much re-design.

Overall your design looks good, the only thing I would change is to move the gantry leadscrew down as far as you can (as close to the bottom rail as possible). Where you have it in the centre of the 2 bearings is pretty much the worst place, compounded too by the fact your rails are very far apart.