Enable-A and Enable-B of H-Bridge L293D (or L298N)
When L293d enable pins are low, the outputs are high-impedance ('Z' on the L293D function table). 

This means no current is flowing through the motor, and hence that winding is applying no torque.

That may be okay when the motor is at rest, and certainly reduces the power use, and heat dissipation.

Further, it is okay for one of the two stepper motor windings while it is turning.

A pattern for turning a stepper motor is to only energise one winding at any instant. This will use less power and run cooler. However, it will produce less torque.

Summary: Yes, switching one winding off, at any instant, by setting one of the two enables low will be fine, will reduce power and heat. However that will reduce torque.