Also keep in mind impedance is not a constant thing-it varies with freq. So if nominal 8 Ohms, it could be a good deal lower than that at spots in the frequency response. So in parallel it could be a good lower than 4. This will heat your amp up and drive it harder. Yogiboy is exactly right, series would be better, not to stress the amp.
BUT Kal's answer above is the right one I think, as now two speakers right next to each other will combine and change dispersion patterns significantly from a single speaker alone. Two wide horiz dispersion speakers side by side (right next to each) become less wide. Two speakers on top of each become less vertical. Think speaker columns-as you increase the driver count vertically you cut the vertical dispersion and widen the horizontal. This works great to shorten "throw" and keep it wide (anyone ever see those gigantic speaker stacks Grateful Dead used to use?). Shorter throw might keep audio off the back wall of an arena, but at home if you stack two identical speakers on top of each other you may notice the speakers don't sound at all the same 8 feet away as they do at 2 feet away.
Also if the speaker is designed to be upright, turning it on its side will rotate the dispersion pattern 90 degrees. What was a wide horizontal/narrower vertical dispersion pattern (good) may not be a wide vertical pattern and narrow horizontal (bad for home hi fi).
Sometimes more is not better!
