The repository (the RPD file) is unique for a given OBIEE instance, so you can't have 2 RPD.
You can perfectly use the same RPD and also the same subject area / business model for your ad-hoc analysis if the data and structure are the same.
In the RPD you can model as many business models as you want, and for each business model you can have one or many subject areas.
So if for your ad-hoc you need something else not present now you can also take that option: extend the existing or build another model / subject area next to it.
So no, no need of a second repository and yes, you can create ad-hoc analysis on the same repository (that's actually what everybody does and what the tool is built for...).