If some one is having some run out problems on one end when turning two, then one at a time is probably best. If there are no run out problems with two parts, then there is no reason not to do both at the same time - except for personal choice only.
I will also say, from my woodworking background, that tolerances are relative. Some people are more sensitive to / can feel and see minute differences in sizes, roundness and at edges better than others. What one would accept and be happy with, another would not. Still, if two parts on a well tuned lathe have not run out, then doing both at the same time would seem perfectly logical.