Where?
I’d worry about prop delay in a series connection as well and you don’t want 2 steppers on the same axis being slightly out of sync.
And doubling your voltage for just one axis is just not worth it and the driver chip itself or other components in the circuit may not be rated for that voltage level.
This is a concern for both serial and parallel hookups, however, it doesn’t seem to bother the community as there are quite a few machines on the low end that parallel steppers.
That’s the limiting factor.
Moving an axis by hand will generate feedback current that can damage the circuitry only if the axis is moved too fast. Slow movement by hand has been proven to be a safe method of positioning the axis in the correct location