One unit per hex does mean greater tactical depth, but tactical depth doesn't belong on a strategic screen.
Another option would be, naturally, to "zoom in" the map when a battle occurs, taking you to a new, more detailed map with all the units that are in "striking distance".
And, as Pikkis said, there would have to be a way to switch unit positions without having to use a third tile.
Oh, I hope they do it that way, but from the screenshots it does't look like it. They don't want a separate tactical screen because that would make Civ too tactical. So instead they make it tactical all over the strategic screen. Its like saying I don't want cheese because it will make me gain weight, so instead you put cheese in a salad and pretend you've accomplished something.
Yeah, maybe they'll have it where you can stack in transit, ie move through a friendly unit, as long as you don't end your move on it.
But if there's still time, the tactical screen could be a hex of hexes, maybe 8 hexes across. On the strategic screen maybe show all units like regiments in the screenshots, but one unit per unit in the stack so you can see them all. So if you have two spearmen units and one archer unit, you have a formation on the strategic screen of two spearmen and one archer. When you go tactical, each of these units becomes a regiment representing how many hit points the unit has left. If you enter the same tile as an enemy stack, all the units are thrown together and they share the hex graphically until the battle is resolved. So you get a red and blue mixed regiment, say if those are your team colors, facing each other so it looks like a battle, and semi obscured by a roiling smoke such as for city unrest to signify battle. Each turn you go to the battle screen (like going to a city screen) for that battle and there are your units and the enemy units and you can fight your tactical battle with archers shooting over spearmen and what all just like they planned, one unit per tile, ranged attacks and all. If you go off the edge with a unit on the battle screen, it corresponds to entering a different hex on the strategic screen. Alternatively, you could have eight tactical turns per strategic turn. After each strategic turn, you go to battle resolution phase and go around to each battle and make eight moves. If not resolved in those moves, the battle is still there at the next strategic turn. And with this you might want a stack limit, so that when a stack enters the tactical screen, which is one hex per unit, all your units can be placed on the edge you came in from. Only modern artillery should have a range into the adjacent hex. The tactical screen represents a battlefield somewhere in the hex, not necessarily the entire hex. Thus it can be randomly generated terrain each time, except cities. And prevent stack of doom by having any unit entering a strategic hex capture that hex, irelevant of culture.