You can pump either one breast at a time with the older models, or more inexpensive models, but there are newer electric models (more expensive of course), that can pump both breasts at one time.  This is quite a time savings if you think about it, since each breast is 15 to 20 minutes of pumping time, so pumping both breasts at once saves you the equivalent of one breast pumping session.

Always keep in mind that if you have an electric breast pump, you want to  make sure you bring a back up battery so that if the power goes out, you can still continue your breast pumping session, so it’ll completely drain each breast instead of leaving you with possibly painful swollen breasts that are engorged with breast milk.