Still early in the year but already a great contender for weirdest question of the year.
Why would you want to sew them together? If you buy a 9 racketbag and it isn't big enough just buy an extra duffel bag. You can wear on on each shoulder and I think it would look better than a homemade 16/18 racketbag imo
If you do however plan to sew them together I would really like to see sone pictures though