If one of your mana bars is falling behind and both Verfire and Verstone proc, you ignore the element you have a lot of as others said until they are balanced.

If Black Mana is low - Use the Verfire > Verthunder dualcast combo. Keep using Jolt / Impact > Verthunder or Verfire > Verthunder procs until Black Mana catches up. Use your Verstone > Verthunder as normal afterwards.
If White Mana is low - Use the Verstone > Veraero dualcast combo. Keep using Jolt / Impact > Veraero or Verstone > Veraero procs until White Mana catches up. Use your Verfire > Veraero as normal afterwards.

Both Verfire and Verstone have 30 second timers. They don't have to be used right away, so you can afford to catch up your bars before using the procs. Plus you get a better DPS gain using your dual cast on long casts since you will still have to wait on the 2.5s Global cooldown.

For the spell AoE, you spam Scatter with 4+ enemies. At 3 enemies, you can use the dualcast on Veraero or Verthunder to speed up the mana bar. For the melee burst, I found Enchanted Moulinet to be more useful on 3+ targets.