I'd guess that the 9.9 will get the job done most of the time but can't
say for sure. 6 hp or 7 1/2 hp will easily plane one adult but is
marginal for two. There's a big difference in planing effort between a
rigid bottom 12 ft boat and a 9 ft inflatable with a soft bottom.

When I was a kid my brother built an 8 ft hydroplane with a 7 1/2 hp
Merc on it. That thing would do 25 mph + but it had much less
resistance than an inflatable and would only hold one person. It was a
mite twitchy at speed to say the least.