I think the people who go first and second should not cast the spells at the same time in PvP. This is because of the following reasons.
People who have good PvP skills tend to go against each other a lot. I have nothing against this. However, whoever goes first always wins. From what I understand, going first is completely random. You might disagree with me, but I think that PvP should be based off of skill, not luck. So here is how it should work in my opinion.
Well, there technically is going first, but in a different way. The first person casts his/her spell, and then the arrow points to the next person. Then that person casts his/her spell, and it points to the next person. This way, they don't cast the spells at the same time. It is pretty obvious when someone is going to attack, so they could easily just shield, weakness, efreet, fortify, ice/frozen/spirit armor, etc. right before they attack! This is why whoever goes first automatically wins.
So, that is why I think that you should make it so that the persons who go first and second don't cast the spells at the same time. PvP should be based off of skill rather than luck, so that we can have fun and not lose every time because we get bad luck and go second every time.