I personally prefer multiple Riders but just two is fine if there are more people in the cast. Just five could get very boring, very fast. That's why I hope the secondary Rider for this show will be a new character and/or that they'll introduce more characters fairly early on (Dr Maki appeared in episode 8 or 9 of OOO and was the ultimate villain of the show, but he wasn't at the press conference or mentioned in any of the promo stuff.)
I think it's definitely possible to do a multi-Rider show without it feeling like a Sentai though. Kabuto did it - Drake and Sasword were in and out of the story at different times, and the Hoppers were antagonists.