Each creature you control with a +1/+1 counter on it untaps at the same time as the active player’s permanents. You can’t choose to not untap them at that time.

Effects that state a creature you control doesn’t untap during your untap step won’t apply during another player’s untap step.

Controlling more than one Ivorytusk Fortress doesn’t allow you to untap any creature more than once during a single untap step.