There are two situations where the matching of multiple motors of the same make should be considered. The first is when they are rotating in the same direction, the second is when they are rotating in opposite directions (Counter rotating).
If two motors are connected in parallel from a single controller, then at all times the two motors will have the same voltage across them. So will they rotate at the same speed under all conditions?
This is an impossible question for anyone but the motor manufacturer to answer properly but with most modern motors any two motors from the same batch will generally be close enough matched for most usual purposes.
With permanent magnet motors the speed of the motor (with a fixed voltage drive) is inversely proportional to the number of turns of wire on the armature and inversely proportional to the magnetic flux density.
The motor manufacturer is not going to vary the number of turns of wire, so the variable factor is the magnetic flux. This is down to the actual magnets and to the magnetic paths – determined by the geometry of the motor, so very closely matched and reproducible in a batch of motors.
So the only real variable is the strength of the magnets. These are not produced by the motor manufacturer but by a magnetic materials supplier and the motor manufacturer will probably know what the variations in strength are. A motor’s speed can be modified by choosing slightly weaker or stronger magnets and some manufacturers do select magnets.
It is also possible to demagnetise a motor by exceeding the ‘demagnetisation current’. See 12v motors on 24v. Permanent magnets are also susceptible to degradation from mechanical shock and from being subject to high external fields – although modern magnet technology has improved to a situation where these effects are minimal.
To check the matching on two motors, connect their shafts together so they must rotate at the same speed in the same direction and apply a voltage. Now measure the motor currents. They should be very nearly the same. Any slight difference in current will indicate a slight mismatch. If mismatching is severe, one motor can even generate a current.
Many motors have offset brushes. This is discussed Brush Offset elsewhere. If the brushes are offset, then a pair of motors that are well matched in forward direction will not be matched if one is reversed.
Since the only way a motor’s off load speed can vary between forward and reverse is if the efficiency varies, you should be able to detect offset brushes by measuring the motor current with the motor unloaded. Compare the current between forward and reverse. Any difference indicates asymmetry.