I know that in general within an orchestra, if the violins hold the melody it's usually the first violins which play the main melody and the second violins act as a support to them, but if the main melody is lower in pitch than the supporting melody would it still be played by the first violins or by the second?
Suppose the main(lower) melody is in the 1st violins, and the 2nd violins double it an octave higher, which, as others have pointed out, not normal. But just suppose it is this way in some piece.
It would not change anything. The idea is that the higher octave should predominate. The reason? If the lower octave dominates, the higher one will be heard (at best) as coloring the lower octave. Why? Because it is itself contained within the sound of the lower octave notes.
If the goal is to have a high melody supported by a lower octave, that's how it should be played. Otherwise, you have the inverse of that.
Now, as to why you would have the higher octave in the 2nd violins and the lower octave in the 1st violins, that is a different matter. If you are the composer, and you do not know why, you should have the first violins play the higher octave instead of the seconds.