You have to press both arrow keys at the same time, sadly you can't do this with just the mouse!
That's strange, are there any squirrels on either side? If they're not rabid and you hit them, it counts as a failure. If there a cut mark and a cut mark with a non-feral squirrel, You hit the mark with no squirrel. However! If there's a chop mark and a cut mark with a RABID squirrel, You hit both marks at the same time!