In disconnected (offline) mode, you can currently only have a single basemap. Currently, you can add additional basemaps as layers as a workaround, with the layer unchecked and the user can check it to turn it on. However, this is a sloppy work flow that would better be executed by utilizing the basemap switcher in Collector. In offline mode, the basemap currently has no use. Instead of loading the additional basemaps (aerials or basemaps) you have into a layer, it should be possible to add them in as secondary basemaps that could be accessed via the basemap switcher. In this way, Collector in offline mode would operate as online mode, but you could access local maps instead of ArcGIS Online maps. Also, if you were online and could access online maps, you should be able to add more than one of your local basemaps.