I believe this answers the whole question easily.
Apprentice Xehanort is quite literally just Terranort with amnesia and two guys having a property dispute inside him who just goes and calls himself Ansem later, while also being essentially the weakest version of Xehanort due to amnesia basically making him the one set to level 1. There's no point in grabbing Apprentice Nort when Ansem, Xemnas, YX, and Terranort all exist and have aspects of him in some form.
That and, once again, he's quite literally just Terranort with amnesia and two guys yelling at each other.