I don't think you ever had the choice. While the rest of the entry for double move did seem to not care when the two moves were declared, the "one speed entry calls it out that one has to declare a double move ahead of time.Count the number of squares you jump as part of your move. If you run out of movement, you fall. You can end your first move in midair if you double move
We can choose to take any action in any order.
So, what happens if I use my move action to jump, see that there's another enemy approaching, e decide to not use another move action?
I fall from midair, abruptly?
Double move said:One Speed: When you double move, add the speeds of the two move actions together and then move.
It is written in a similar way in regards to stopping ones movement in an allies space in the double move section, You can 'end' the first move illegally because you are not stopping.The Jump rules seem to contradict that, as they say you can end your first move in midair if you double move. If double moving was a single action, you couldn't end half of it like that.
Occupied Squares: When you double move, your first move action can end in an ally’s space, because you’re not stopping. Your second move action can’t end in an ally’s space, as normal.
Man, this is confusing... anyway, here's how I intend to handle such things. Not sure if it's RAW, but it seems like the simplest and most logical approach.
When you jump, you first declare where it is that you intend to jump to. (You can also declare that you're simply going to jump as far as you can in a given direction.)
When you declare your target, it must be one that you could reach with either a single or a double move. You then spend a single or a double move action, as appropriate, and make your Jump check. If the Jump check is enough to get you to the target square, you land there. If not, you go as far as you can, then fall.
If something interrupts your single or double move action and limits your movement so that you can no longer reach the target (e.g., an enemy fighter whacking you on the way past), you go as far as you can, then fall.
You cannot, in midair, decide to abort your jump and fall. Having committed to the jump, you must keep moving toward your destination. Contrary to popular belief, this is not a video game.