That's so when only tapers and plain turning is involved, but not so when all aspects of turning is involved, such as Circular Interpolation. In this regard, the whole package must be capable of Trajectory Interpolation across mixed axes, that is X for one spindle and Z for the other spindle.
There are many MTB that offer such machines. Mazak have a Twin Spindle (in various sizes) with at least two linear axes per spindle (basically a Left and Right Hand lathe joined together) with a function labeled Swiss Mode. In this Mode one spindle provides the Z axis and the other the X, with true interpolation across the two axes. This is not possible with simple electronic gearing that you're referring to.
Hello Ox,
Its not all that simple. A lot of twin spindle, multi axes machines don't precisely know what each axis of one spindle is doing in relationship with each axis of the other spindle. Accordingly, without electronic gearing, or the capability of Trajectory Interpolation across typically unrelated axes, the best you could do is rely on timing between the two spindle axes to produce a result. Plain parallel turning would be possible, but anything past that is a fudge.
Regards,
Bill