There is no reason, no evidence, to state factually that Sonic will be given a 2nd character. None. Stating that without proof of any sort is simply blowing smoke.
If Sonic were such a big deal since Brawl, then why was he added last minute and not the first 3rd party revealed instead of Snake? Logically, Sonic would have been planned from the start and been the first to be shown.
Because it is a Nintendo crossover, I myself do not believe that any 3rd party series should get a second character. Once that happens, that means that that series is more of a big deal to Nintendo than F-Zero and on par with Fire Emblem and Mother, and Yoshi, and Wario...that's not believable.
I love Sonic, and I love his series. He's not a Nintendo character. If Nintendo owned Sega, then it would be a different story probably. But until they do, I'm rather certain Sonic will remain as the only Sonic character in Smash Bros.
No need to get heated in here guys. It's all just speculation.