Full size breakers vs. tandem (double) breakers?

Stefonius wrote:
If you're running "one heater on each leg", you should use two single breakers rather than a tandem. You can easily make sure each breaker is on a different leg. If you're using a tandem breaker with two potentially unbalanced 110v loads, you can cause problems.

Taking the single out and adding a tandem will put each heater on its own leg. He will not need two single breaker since the tandem will give him two legs/circuits. Balance will be no different than having two single breakers.