We used a sock on an 18.5 ft bell Northwoods and ran it off the stern and it slowed us down very nicely. Enough for us to jig up some eyes. I would be nervous about deploying both off bow and stern. That would put you sideways to the waves AND if the waves are big enough to make you want to deploy a sock in the first place you could be asking for trouble.

If it were me I would just get a cheap small sock, add 8' of line to it and tie it off at my seat. Remember you have to bring the thing in again eventually to. This would have have the seas running more or less into your stern which would be safest. Even a small sock will peg your boat plenty. Get to big a sock and you might not move at all.

Good luck Kerry. Let us know how you do.
P.S. The yellow socks have been known to attract pollyhumpers so be careful.