Of the couples who show up fairly regularly at the local lake, most have individual kayaks. As with Apalach, they are usually in different models, so there's more choice and diversity, whether going together or by yourself.
Of those in tandems, there don't seem to be that many regular
twosomes for some reason -- lot of one-timers -- and more often than not, one person is doing most of the work while the other one sits there bored or cold. (of course, these are all paddling kayaks -- don't see too many Mirage Tandems).
When the wife goes, she likes to be in the tandem, but she doesn't go all that often. Come to think of it, the last five times the Tandem has been out, I was with OffRoad
, my daughter, a neighbor and myself twice!. Going solo, it's nice to have a single -- easier to handle, faster, better balance.
Disadvantages of the Tandem -- 1) With both facing forward, it's hard to have a conversation. 2) My wife doesn't like the paddle flinging water on her, so I don't get to paddle (exercise value only). 3) It doesn't carry much gear compared to the singles. Advantage -- If you're a lot different from each other in speed or endurance, you don't get separated in the Tandem (you never have to wait for your partner!). And it's always fun to go together. The Tandem also offers unique advantages if your partner is disabled, uncomfortable in the water, or you want to bring a dog or young child, for instance.
So it really depends on your situation. Like Apalach, I think the single route offers the most versatility and works out best for most
people. If you get two at once, you might be able to do some serious negotiating on price!