Hello Thrilledtobuild,
Building the field tile out should not be under consideration. These issues should be addressed at the time materials are being selected. Unless your installer is giving you a rendered wall (mud job) where he can carve out a channel for the accent, I suggest that you use a decorative relief to isolate the two planes. A simple half-round liner used above and below the accent will define the detail while also obfuscating the 1/8" disparity between tile thicknesses.
Best of luck,
Shaughnn

When DH put up the cement board (Hardibacker) on our hall bathroom tub surround walls, he used a thinner piece of cement board where the border/accent tile was going to go, creating a channel of sorts for the thicker tile.

Problem is that causes weak points in the wall, where that thinner cement board meets the thicker. Additionally, the 1/4" cement board should not be used on the wall, anyway, because it's not strong enough. it'll flex too much.

Schluter profile strip. (search this.).
It can cover the edge.

It (another profile) can also follow the 3/8" tile, and I think this is what one responder mentioned already: "... use a decorative relief to isolate the two planes. A simple half-round liner used above and below the accent will define the detail while also obfuscating the 1/8" disparity between tile thicknesses. ..."

Apparently he did NOT do it that way so ignore my post.

Thank you all so much! I had the meeting for the layout with the tile guy today. He wanted to just let the marble border stick out a bit more from the wall. I don't like that you can see the unfinished edge of the marble, so I'm running out to find the decorative trim piece Shaughnn spoke of. I am so grateful for the help and suggestions!