Ok guys, thanks for your advises.
My 8.4 is a 2002 model (in fact, a late 2001 model, with 7 balloons), and I usually use it from 17 to 27 knots with my Fone board, I think from 14 to ??? with the Olry 186.
I search something for the very lights conditions, because I will use it principally on our swiss lakes (where we can have less that 10 knots...).
Do you really think that by combining my both boards, I would have a gap beetween the 12.5 and the 8.4 ??? I heard it was easilly possible to hold the kite until about 20 knots... But I just could test this size (12m) in 5 to 8 knots...
If the 12.5 can make me win 3-4 knots, it will increase my navigation time for about 30% !!! So, each knot is important
ONce again, thanks for your advices !!!
Have a nice WE.