Check out what Stringy does to get to work on this post viewtopic.php?f=32&t=19322
, just what you want to do I think.
Like Matt says, the side kicks are probably your best bet, but if you want to try something different read on.
We have been sailing Kayaks for several years now (our favorite thing to do), we started with an Oasis, and a Revolution, had lots of fun. If you go to the forum thread The ultimate Tandem Island (hydrofoils,spinnaker,jib,etc) viewtopic.php?f=69&t=33720
You will see about halfway thru the post, a keel weight setup I put onto the boats to make them a little more fun and safer to sail (basically you can't tip over with them in). The weight basically does the same as sidekicks, we also had jibs and spinnakers rigged on the Oasis and the Revo. The keel weights do add a little drag, it's pretty close to what you would get from sidekicks.
If you make the keel weight shaped like a torpedo around 25-35 lbs and suspend it under the boat with spectra line, it all works great. It's easily raised up for shallow water, launching, and beaching. Actually with the torpedo shape we don't do anything special when beaching, it's a hunk of lead, you can't hurt it. The easiest way to make is just go to any dive shop and buy 25 lbs of weight belt weights (the kind made from lead pellets in a mesh bag). Then take a 24 inch piece of 2 inch PVC tubing and just fill it up with the shot. Glue two end caps on the ends and your good to go. If you want to get fancy shape a bull nose in the front and a point on the back, but they won't improve the drag a whole lot. Tie the spectra around the tubing near each end with cinch knots. The end caps prevent the spectra from coming off. The spectra (same line used on the rudder lines, available from Hobie for $.29 /ft) is good to almost 500 lbs so it will never break. 24 inches of 2 inch PVC pipe is good for 25 lbs (good for revo), Oasis would need 30 to 36 inches length (ie.. 33-40 lbs)). A 36 inch long 2 inch PVC pipe is around 40 lbs (good for Adventure). The PVC pipe is cheap under $5.00. But the lead will be about $75 bucks for 25 lbs. Easiest way is to just pour the desired weight into a longer than needed tube with one cap already glued on, then just saw it off and glue the other cap on (easier since you are buying the lead by weight). If you decide to change it adding more weight later, just get a new pipe and cut the old pipe open and re-pour the lead plus the additional lead into the new tube.
I sailed our Oasis with the 25 lb keel weight for several years, but wished I had 35 lbs. The Revo worked great with just 25 lbs (impossible to tip the boat over). My TI has 50 lbs and is impossible to tip over without the sail open, but when you open that huge 90 sq ft sail you can still tip in anything but really light winds without the AMA's, I will probably add another 25 lbs (probably a second torpedo type keel weight) to the TI so I can sail without the AMA's in regular conditions around here.
I mostly just use the swinging keel weight (if you wanted to get really fancy) on the TI in heavy sea conditions (20-30 mph winds and 3 ft plus seas), the rest of the time I just leave it in the car. I was out last weekend with the 50 lb swinging keel and AMA's on my TI in 22-25mph winds with 30mph plus gusts sailing, it was actually fun.
Good luck and have fun