One thing to search for is 'bimini', the term for a boat cockpit cover.
I know of the WindPaddle version, it's on their main page:http://www.windpaddle.com
A friend has one and likes it, I've thought about getting one myself but haven't yet. I'm not sure how it would work out on a tandem - it might be inconvenient on the front seat. Folds up small and (in theory) can be quickly dropped in case the wind comes up.
A quick search actually brings up "Hobie bimini" - looks quite interesting - but I don't see any reference to it on Hobie's website, just in the forums. Looks like it was made by Pro-Techt and it's still available:http://www.basspro.com/ProTecht-Portabl ... uct/61168/
I've also seen quite a few people just build a PVC structure behind the seat with a post at the top to drop a golf or beach umbrella in. Perhaps, with a PA, just a fishing rod holder on the rail would work? I'm not sure how much loading those holders can take - any wind and the umbrella would need a strong mount.