I'm using Aston at the moment but I want to try LiteStep and I'm just wondering, I want to keep Aston in case I don't take to LiteStep so will it be ok to just disable Aston or does it need to be uninstalled completely?
Also would would happen if one was to actually run both together?

I think it should be fine to just install Litestep and if you don't like it you should be able to use SetShell to change back to whatever you want. Maybe someone else can comment on how well swapping shells with SetShell works.

If you run both Aston and Litestep together your computer will burst into flames.

What you may want to do is install an application like ShellOn where you can choose which shell you want to load on startup. That way you don't have to commit to one or the other but can choose each time you boot the computer.