Many speakers have a '
pattern' with differing horizontal and vertical coverage, you shouldn't mount any
speaker sideways unless the resulting
pattern works for the application. With multi-way speakers as suggested by the reference to low and high frequency drivers, unless the drivers are in a coaxial arrangement, even conical
pattern drivers will result in some variances in the
pattern through
crossover. You also have to consider nearby surfaces and how those might affect the resulting performance. So there is no way to make a 'right' selection from among the provided choices without knowing the specifics of the situation and my choice would be "it depends, wouldn't do it at all unless the results made sense".
If the only reason for mounting the speakers sideways was that the speakers wouldn't physically fit in their intended orientation then my choice might be "select a different
speaker that better fits the application".
If they were mounted sideways because that provided a desired result, then I would tend to go with a mirrored arrangement, but whether it was the high frequency drivers toward the inside or outside comes back to "it depends".
