As was noted at the beginning of this guide, tandem kayaks can still be used by only one person. Well, some of them can while others are only optimized for two. Anyway, the best control you are going to get when riding solo is in the middle. If you want to consider other option for solo, take a look at our guide on SUPs.
If there is no middle seat or you can’t move the seats around, opt for the backseat. In the rear, you should still have a decent amount of control. However, do not sit in the front as you will have inferior control of the boat.