just had my first powered up session with the 12m Rebel 08 (with the 07 bar).
I'm something around 60 kilos. It blew up to 30 knots today and I was still on my 12m. Hanging on, but still going. I opened my QR at 35.
No way you'd get overpowered with a 12m on a surfboard before 18-20 knots... If.
So a 12m/9m combo should do just fine.
My 12m gets me planing at 10-12 knots, a 9m would reasonably cover 25 knots + (but would get you going on a surfboard in much lighter winds). I believe most of us stop kiting a 40 knots + so you should have your wind range covered just fine...

The combo should also allow you to have your different styles covered (between wave riding and freestyle), as the 12m rocks on strong wind (and you will love having it then).
Holy smoke this baby jumps big...
And still can't believe the wind range of my 12m...
I'm so in love with this kite.