I want to DIY.. some questions please!

[Jonathan]

Hmm, surely the parallel connection will draw twice the current of a single winding, not twice that of the series connections. Similarly the series connection will draw half the current of a single winding, not half that of a parallel set-up.

In parallel the current is twice the current for one winding. In series it is the same as the current for one winding as you have the same current flowing through both.
GCSE Physics...

I still wonder though; why isn't it better to use the same screw pitch for all axis, so that providing the motors don't run out of torque, each axis will have the same speed and resolution?

Each axis has a different mass and length of ballscrew, both of which affect the feedrate. So the same screw pitch on both will result in different speeds since other variables are still different. The resolution will be the same.

Your Y-axis is 600mm (?), so I'd still go for 10mm pitch on Y.

I really don't want to go the rotating ball-nut route

Why?

My question was really about why it is better to power each motor and ball-screw from a separate drive circuit, rather than just using both motors from one circuit. The motors can surely still individually stall, even with two driving circuits.

Same answer, but to put it bluntly if one steppers stalls when you have two on one driver then you can say goodbye to the driver. If one stalls with two drivers it's a bit annoying as the gantry is bent, but not the end of the world.