Yes, like Matt said...In the case of a circular diaphram blumlein pair you can mount them horizontally, although the bodies will be hanging in the back lobes and would cause more diffraction and interferance to the rear sound field.Sometimes There "can" be phasing issues, milliseconds of delay with side by side placement,but horizontal mounting will work. In the case of something like ribbons(or Pearl ELM mics), where the capsule is very thin and long, horizontal mounting is bad because the sound arrives at one end of the ribbon first and travels along the length. What a lot of people forget is that directional patterns are 3 dimensional.