In my store we don't share tills and it's still a problem. Your average person will take in €1000 a day, and if you're on tills all day it can be over €4000 at Christmas. They start highlighting overs/unders when they reach €3 and you can be cautioned and warned over them if they continue.
By all means scan stuff and pack them into bags fast. When you have the note in your hand, give it a quick glance just before you enter it into the till. Another thing we were always told to do is put the note you've taken on top of your tillbox (ie. on top of the cash but not put in right) so that you'll know what the change is and if there's a dispute over what the customer gave you, you'll know immediately. If the customer arguses they gave you a different note, like a 20 instead of a 10 and you can't be certain, NEVER GIVE IN. ALWAYS get a manager to count your till. Chances are they won't hang around for that anyway.