That's normal. In fact, you should be re-adjusting your position a whole lot more frequently than that ... like about every 10 minutes or so. That's one of the issues with riding a tandem, you have to hold still so that you don't throw the tandem all over the road. I can hold still for about 10-15 minutes, and then I've got to move ... so we coast and stand and stretch etc. about that frequently.

On your single bicycle you're free to move whenever you want ... and it is a good idea to stand up and stretch periodically, or at the very least to shift in your saddle and readjust your position.

And about every 1-2 hours, depending on how you feel, it's not a bad idea to hop off the bicycle and do a bit of stretching.