Osmand-nightly = Osmand+
Osmand-default=Osmand Free
If you have OsmAnd+ installed, you can't install osmand-nightly over it. It's the same version but the Play version is a signed version and this nightly debug is not. They conflict and therefore you can't install it. This is to prevent hacking of official apps.
If you want to run 2 versions next to eacht other, you need to install the Osmand-default next to your Osmand+
Or install the Play Free version next to a Osmand-nightly.
For testing it doesn't make a difference at all. You don't have wiki in the Free version and a limit of 10 maps. simply download the maps in your Osmand+ version and install OsmAnd-default as nightly build next to it.
Harry