Kind of, you would have to merge one into the other. The skeletons would have to have different bone names though otherwise they would both end up using the same one.
Couldn't you could export prop mesh as an .obj file and load it in, create your animation with it, and delete it before exporting your finished animation?
May I suggest using an 'attachment' to a single bone added into the Scene? This is how I approach an 'opponent' or prop needed in the same scene as my working rig. You can even animate it[in it's parent Scene] and have your currrent rig react to it. This way, there is only one extra bone object in the current Scene, you get a very nice separate object to render as you wish, sync animations to as you wish, scale as you wish... . The 'feature' is already there...in a manner of speaking!