We always call the tandem kayaks "floating divorces" =)
Do y'all have much experience kayaking? If so, ignore me!
I find when one has more skill than the other, a canoe is better - the less-skilled one can be in the front, and provide some additional power when needed, without impacting the overall function of the steerer in the rear. In a kayak, your timing has to be better, and you have to be more synchronized, otherwise the one in the front should just pull up their paddle as the difference in steering ability is minimized between the two... Now, some people make it work just fine!
I kayak a lot, and my lady rarely - she generally has a bad time and feels less comfortable/secure in two-seater SOTK (sit on top kayak), but really enjoys canoes and hybrids with me. To be honest, it may just be the presence of a gunwale with her.
If you're going with a 'yak, I'd start with a SOTK rather than a SINK (sit inside kayak, i.e. the ones you use a spray skirt with) - especially if going tandem, they're easy to do recovery and wet entries in, and with two wet entry is much easier. They're also way better for fishing (we use SOTs pretty much exclusively down here for bay/marsh fishing) - it's not uncommon to see guys rolling through the bays down here with a cooler, live well, and a truckload of gear on their 'yaks. Lol.
I'm a big fan of Wilderness Systems - I have an older Tarpon 160i I've used for nearly seven years that has stood up to abuse and saved my life on a few occasions! Echoing what was said earlier, shorter boats are better for small rivers, but since you're not white watering it, you'll be fine with nearly anything.
Above and beyond all SKIP the Academy specials, Pelican, etc. are nothing but leaky vessels waiting to put you and your family's health and life in danger. Buy something reputable, or get used to bailing water.