It's not recommened but it should be ok if you set them up correctly.
Here is something I found from a site:
"If you sway from side to side in front of a vertically aligned box (ie, tweeter over the woofer), the distance the sound travels from both drivers to your ears remains unchanged for a wide sweet spot. When the box is on its side, it's common for you to be positioned closer to one driver than the other. Where the two drivers overlap and share sounds around the crossover point, the separate soundwaves combine slightly out of phase from each other. With a 3kHz crossover point, the wavelength will be around 11cm. This means if one driver is heard from a point that1s 5.5cm further away than the other, the entire frequency will disappear due to wave phase cancellation. Incidentally, some manufacturers recess their tweeters into the cabinet for better acoustic alignment with the woofer."
It's got to do with the delay of not hearing both sounds at the same time.