This extension will put 2 icons at the bottom of the GMs combat tracker.  After rolling the initiative for everybody, the Alternating Initiative button can be pressed to change the order of the combatants to follow The Simple Initiative System described here:

Taking20 Youtube video describing the Alternating Initiative idea

The combatant with the highest initiative will go first, then the combatant with the highest initiative of the opposite faction will go, then the next highest of the first faction, etc.  alternating between factions.

Hidden combatants will still be put in the alternating order, but will be given an initiative number allowing them to still be hidden from clients.

Combatants of the same faction that roll the same initiative will be lumped together.

Also, if needed (say if a new combatant will be entering the mix), the GM can hit the other button and the initiatives will be reset to the original rolls.

Update 2.0:  Added an option that allows the GM to pick whether or not to include hidden characters in the alternating initiative.  If the option is set to "No" these hidden characters inititive will be set to 0 so they are not in the middle of the alternating order.