There could be some people here who can help with tandem kayak advice, but you'll probably find more help on the paddling.net site, because many users here are whitewater kayakers. The camping and flatwater kayakers tend to go here:
www.paddling.net/message/showTopics.html?fid=advice
There's also reviews of tandem boats on that site, here:
www.paddling.net/Reviews/
In my experience, most kayakers don't recommend a tandem boat -- they're known in some places as "marriage breakers."

Although Jackson has just launched a tandem whitewater kayak, which some people seem to love:
www.jacksonkayak.com/kayaks/product.cfm?product=dynamicduo
But you might want to look at the new Native Watercraft tandem kayak, which has removable seats, so you can make it a solo boat with lots of room just by taking out the second seat:
www.nativewatercraft.com/marvel_14T_lg.cfm
All poly tandem boats that I've seen are heavy. Performance in the water is not something you're likely to find in a tandem boat, except maybe the Jackson Duo. You'd have to get into expensive composite boats to get any performance, and then you're not talking about a boat you'd put your dog into. . . .