Depends how far north you are talking. Once you get past Turtlehunters territory (Exmouth) then you will be needing 12M and up. A big ask wanting 1 kite for 2 people different weights when you are talking Nth in WA. its a BIG coastline.
You will need to cover a huge wind range with only one kite. If they must take just one kite the Cab Crossbow has probably the largest wind range so yes a 10 would work, and they could take it out in some very strong winds it their board skills are good. Trouble is they will then have to kite with a Crossbow
