hi Roger. if the two negative lenses have the same fl (not necessarily the case though), then the different magnifications depend on the positions of the negative lenses relative to the prime focus of the main optics and the different spacings between the negative lenses and the camera chip. ....which basically agrees with your statement.
Worth a read if you have not already come across it...
http://www.brayebrookobservatory.org...BarlowLens.pdf