Alberto, using two center speakers in parallel isn't common, but isn't extremely unusual either(when I first got my M22s I briefly experimented with using them in various center channel configurations). Of course when it's done usually the two speakers would be identical, so in that respect your setup is quite unusual, but if you like it, fine. How are they arranged; horizontally, vertically, etc.?
You calculated the parallel impedance correctly, using the nominal 8 and 6 ohms( 1/X = 1/8 + 1/6 = 7/24, X = 3.43), but of course at some frequencies the actual impedances drop below the nominal figures. Apparently the 770 still approves.
