I share a quiver with my girlfriend that is 60 pounds less than me. Usually we are 2m appart on sail sizes. We like riding waves on a TT ( Nomad wave).
Last trip in Cabarete I mainly used 9 m and my wife 8. Last year we had 8-10-12m and would often wait for the 10 to come in as riding 12 in waves is a lot of work. My wive would then jump down to 8, adding the 9 was great.
Now the question, we are both going to Brasil this fall. Was there last year by myself and mainly used the 9. Was 8 on strong days and 10 on low days. Our quiver is 6/8/9/10 ozone enduros. If i ride 8 my girlfriend must go to the 6. Its a good gap, but then again in stronger winds it may do.
I am challenged between bying a 7 or start managing line lenghts. My girlfriend could from 8 with 20m lines go to 6 with 24 or 25 m lines. This would bridge the gap, and avoid bying and carrying another sail.
The line change does make a lot of difference, for me at least. I was in Ceara (Ilha do Guajiru) last month, with a 8m xr5 and a 10.5m wainman. I could easily have used a 7m most of the time, and one or two days a 6m would be the perfect choice, but i managed to ride every day during my entire stay (8 days) only taking the lines extensions off from my 8m. The 10.5m ive only used during a half of one day. During two days was blowing 28-34 kts.
Ive tried line lengths of 20m and even 18m. Felt better with 20, but both worked very well. It took me some time to get used to the diferent lengths as i had never used a diferent line length from 24m before.
Fyi: I consider myself a beginner-intermediate kitesurfer, my weight is 66kg and I rode only a twintip.