Is It Ok to Inject a Double Amount of Filler into the Lips at a Time?

The amount of volume each person needs, and wants, is different. It is not unusual to use 2 cc's for lips. If the result is what you want, then consider returning for a touch-up before all the volume is lost. This way you keep the look you desire, and will still have some Restylane left in the lip to build on. It is likely that you will only need 1 cc at a time for maintenance. If wait and need two, it is fine to do 2 cc's. Most experienced injectors will do 1 cc the first time if you have never had lip enhancement before. This is to prevent over correction. You can always have more added, but removing too much volume is a harder problem.

Different people need different amounts of Restylane, or any other filler.  If 2 cc seemed to be the correct amount for your lips, it would be fine to have that injected at the same treatment session.  The amount used should not cause a higher chance of unevenness although it is possible that you may notice a bit more temporary swelling.