Sea Kayaker published a review of kayak sails back in 1990. Does anyone have any newer experience and/or recommendations for a sail for a double kayak?

The 'Spirit Sail' works great on my single and I don't see why it would be any different on a double. Two sizes...hands free. Very stable. Always use your rudder when the sail is in function.
Just google it.

We also did a review of several kayak sails in the April 2005 issue of Sea Kayaker. And in the August 2007 a review of the Feathercraft Windseaker sailing rig.
And in the December 2009 issue a review of the Kayak Sailor 1.4 complete sail rig for kayaks.

I've tried a few different sails and the great thing about the Spirit Sail is that there are no ropes (hands free) so you can paddle without any worries. Also very fast to put up and take down. The mount also works great for a trolling rod for salmon, etc.

Any reviews about the Kayaksailor 1.6 meter sail? I have a 56 in WindPaddle now and would like to sail into the wind as well as downwind. We have a tandem kayak.

Frank, Sea Kayaker did a favorable review of the 1.4 Kayaksailor rig on a single. Talking with the reviewer he felt that that review would probably answer all your questions about performance, setup etc. The review can be found in the Dec 2009 issue of Sea Kayaker.