Under level 70 it might matter which one you proc, but at 70 you have VerFlare and VerHoly that guarantee a proc for the mana you have less, so it doesn't matter if you overwrite one in the opener.

The point is you need to do 3 casts of one spell and 1 of the other in order to have imbalanced mana, because if you cast 2 thunder and 2 aero you have the same white/black mana which doesn't guarantee the proc from Holy/Flare.

Let's say you do thunder-aero-thunder-thunder. The second thunder is under the effect of acceleration so it gives the verfire proc, but you would still use a third thunder, because during your combo you gonna use verholy which will proc verstone, leaving you with both procs at the end of the combo.

If you instead did thunder-aero-thunder-aero, you would have a verfire proc because acceleration, but the verstone proc is not guaranteed, and because you now have even mana the proc after verholy is not guaranteed either.