Robin2:
I have been under the impression that the current specified for a stepper driver is the "per-coil" current so (in theory) a driver that is described as being able to supply 2 amps should be able to provide that for both coils (i.e. 4 amps).

Am I wrong?

Yes, its a quadrature current rating, so the sum of the squares of the current in each coil is a constant.

Or put another way

Ia = I sin(theta)
Ib = I cos(theta) [ theta being the "electrical angle" of the motor ]

For coil currents Ia, Ib where I is the nominal current. The maximum total current from the driver is when
theta is 45/135/225/315 degrees, which each current is 0.707 I, summing to 1.414 I

At angles 0/90/180/270 degrees, one winding has I and the other zero.

With full steps many drivers use 45/135/225/315 degrees, for say x4 microstepping the angles would be
0/22.5/45/67.5/90/115.5/135/.... So the current settings vary over a range of sin/cos values as steps
happen.

The total heat dissipation in the motor due to winding resistance is constant as the sum of squares is constant,
but the dissipation in the driver chip depends on the angle.