I have only used D-23 diluted 1:1. The purpose of the second bath is to allow shadow density to build without impacting the highlights. The theory is the highlight density builds up in the developer and exhausts itself. Placing the film in a bath of borax or metaborate allows development to continue in the shadow area.

Adding the borax to D-23 really converts it to a variant of D76 called D76H and you would loose the compensating effect.

I have tried divided development in the past, I used divided D76 and while it worked OK I saw at least a full stop in film speed. I used this when photographing the inside of church buildings which had a very wide contrast range.

Hope this helps

- Mike