JR XP662 and I have a JR 8103. >I would like to mix in Flaperons with my 8103. >can my boy's 662 still "drive" the 8103 on a buddy cord?
Short answer = yes.
Longer answer. The 662 supports flaperons. (ref: section 6.2, page 25 of the 662's JR manual.
In buddy mode with the 662 as master just make sure both Tx's have flaperons selected under wing type. Suggest you both set up a specific training model number with the same name & profile with flaperons enabled.