As someone said above, you COULD use the power twice during your turn if you want to give up your standard action and then have a choice of two targets to trigger on, though you can obviously only actually trigger on one of them. Note too that you can trigger on ANY square of movement, so you don't have to use it on the target's first square of movement for instance. This could be handy if for example the target moves out of your line of sight. Say they go around a corner at some point in their move you could wait for that to happen, trigger on that square of movement, and that way stay out of sight. This brings up the final point, as soon as someone moves to a spot where they don't have cover they are going to lose their hidden status. The DM might rule in a non-combat situation that the target isn't looking your way, but in a fight you're not necessarily hidden as you move.