Will One Syringe of Restylane Plump Both Lips?

This really depends on your desires.
Typically, I recommend no more than one syringe the first time.
You will be able to judge the volume of the correction at the completion of the injection:

At this point, you will be slightly more swollen than your final result. If you complete one syringe and desire more, than by all means have more injected at that session while you are still numb.
If however you feel you are too large, relax, because it is slightly swollen and will likely reduce in size over the ensuing week.
If you do not like the look at all or the balance of the upper to lower lip, the volume can be adjusted or "undone" using hyaluronidase in an "off-label" indication.

Generally speaking, the lower lip is larger than the upper lip. However, many patients prefer a fuller upper lip. I therefore typically use 0.6 cc to the upper lip and 0.4cc to a lower lip using a 1.0cc syringe. I can easily adjust this to patient's desires or anatomy.
I hope this helps!

That is a very good question. This can vary from one person to another. If your lips are very thin, you might need more than one syringe. If your lips are mid sized or fuller you might even need less than one. The best thing to do is to get evaluated in person. In my experience usually patients need about one to two syringes to add more volume to the lips. 

One syringe can be sufficient to enhance your lips, but it really depends on how much you’d like to “plump”. If you want dramatic improvement, one may not be enough. I suggest you start with one, and if it’s not enough, then see your injector two weeks later or so for another syringe.