Well, I'm admittedly somewhat a "precautionist."
You have to learn a new charting system, where everything is, who is who, and, well.....how they roll (that latter thing being what will make or break you if you don't figure it out in whatever time frame they consider reasonable).
Two weeks is still only 6 shifts. I don't think there's a lot of harm in asking for a couple more shifts up front. If necessary, you could say something like, "It may turn out that I don't need them, but it would take off some of the pressure and allow me to focus on making sure I'm doing a great job in my new place."
ED people are go-with-the-flow people, so I know my caution sounds wimpy. It's just meant to be food for thought.
I would really hate for them to run with the assumption that you're a big shot w/ this Level 1 experience - - for some reason that is a set-up for scrutiny and very high expectations.