I use a Nikon 2X teleconverter. It only works on my 70-200 and my cheapie Opteka 500mm lens. It does not work on my 70-300 or any of my other lenses. it just does not fit on. There may be an adaptor. It has a very good glass and works well.
Which actual 2x tele is it though as there are many available. Is it perhaps something like a TC-20E II...???