My gym bag is a single pannier, I bought it as a single. Often ride with one that one.

Several different manufacturers of panniers sell touring panniers as pairs, but sell shopping panniers as singles. I can't remember the last time I went to the grocery store with two panniers, but often use one for groceries.

Several comments above said that you have to try hard to get your front panniers balanced. I used to think that too, but one time on a group tour I overslept. I did not want to be really far behind the group that day so I just threw stuff into the panniers haphazardly without trying to balance weights. At the end of the day, I realized that my bike handled just fine so I weighed the panniers with my luggage scale to see how badly the balance was. (I carried a luggage scale to make sure I did not have over weight charges for my trip home after the tour.) Weights were:
- 14.48 lbs - Left Front Pannier
- 8.88 lbs - Right Front Pannier
I did not notice any handling problems or pulling to one side. Perhaps it was the bike (Thorn Sherpa) or something else that prevented the odd weight distribution from interfering with the handling? All I can say is that I have since stopped trying to make sure my front panniers are balanced.