I'll have a crack Vin
, but remember I've been wrong before
The way I understand it is the when you add a 2X barlow to a scope you are effectively doubling its focal length, not halving the eyepice Focal length. Therefore a 1000mm scope becomes 2000mm. So in the first a 25mm eyepiece will gove 50X magnification and in the second it will give 100X magnification. But keeping the same eyerelief and exit pupil (?) as the original 25mm. The downside is increase optical elements in the optical train.
However for ease of explaination its often expressed as halving the eyepiece focal length, rather than doubleing the OTA FL, simply because we are used to expressing increases in magnifiation as a function of eyepiece focal length.