I want to use some Dura Ace outer cable with some XTR inner. Apparently its for use with 1.6mm inners, but I’m not sure how thick the XTR inners are, these aren’t XTR but they are MTB and they are 1.6mm…

OK, I seem to have answered my own question here…

Yes, although brake cables are usualy sold as universal, then you cut off one end for road or the other for MTB (Road has an axial cylinder on the end, mtb has a radial cylinder).

Dunno if XTR are like that, so they may not work in a road bike.

The outers are interchangeable, have been since the 2.00mm brake cable died a death (1990s sometime). The inners you just need to make sure they have the right barrel to fit your lever as per ^^^^