Initiative in combat is a weird thing in role playing games.
It's pretty much impossible to model the simultaneous and chaotic movement of a fight.
D&D and a ton of other games do it the easy way, by having people go around the table in some order, each taking their action.
This approach is super simple but always bugs me when I think about it. Granted, I'm not trying to completely accurately model a fight in my D&D game, but sometimes I do like trying to model the chaos and confusion of it.
Barring being able to actually have everyone do simultaneous movement and action, I have an idea or two about ways that I could do it, in a Labyrinth Lord game, for instance.
Using group initiative, when it is the party's turn, they go in order of Dexterity, highest to lowest. Even better if you've seated them around the table in that order. The player with the highest Dex either declares his movement and actions, or holds. The player with the next highest Dex then either declares her movement and action, or holds. A player that has held may then declare their movement and action after any other player has completed their declaration. Thus, if Frank has the highest dex, followed by Suzie, Joe, and Sam - Frank can hold, then Suzie goes ahead and declares her action, and Frank then jumps back in and declares his actions. Now, bear in mind that we're just declaring actions. No one has moved yet. That all happens in order of highest Dex again, just as outlined above, but declarations all happen before any moves or actions. And you can't deviate. You either do what you said you'd do, or do you don't.