I've just been doing some experimenting to confirm what is being said and to perhaps clear up some confusion.
Remember Short addressing uses CV1 as the loco address. This is basic addressing and should really be dependant on the decoder. Lenz Gold can have a range of 1 to 127.
Long addressing uses CV17 and CV18 to determine the high and low bits of the long address. The range of CV17 is 192 to 231 and the range of CV18 is 0 to 255. CV29 has to have a value of 32 added to it to turn on long addressing. This is all done and calculated automatically when using a system capable of handling long addressing such as the Lenz system 100 with the LH100 hand controller/programmer.
When using the Hornby Brit, I've programmed the long address to '7003'. The CV1 remains at the factory default of '3'.
Above, the loco can be controlled by the Lenz LH100, but not from the Arnold 86210 controller.
Here, I've set the CV1 of the loco to a value, then specifically back to '3'. Setting the CV1 seems to turn off long addressing with the Lenz system. Now the loco is controlled at address '3' by both the Lenz LH100 and the Arnold 86210 controller.
So it seems that in this case, you can't keep the long addressing on a layout that uses short addressing too. I thought that perhaps the short address controller could use the CV1 setting and the long address controller could use the long address of CV17 and CV18, but alas, that is not the case.
I wonder how the Hornby Select and Elite are going to function together. If they both use a shared database for long alpha-numeric names then that's great, but that won't be compatible with other Xpressnet controllers.