20 Mule Team Borax from the laundry section at the grocery store is the decahydrate. I've heard people recommend against it as possibly impure, but have not heard of anyone reporting problems with it in developers. I have used it myself in both the clothes washing machine and in film developers.
I'm not sure that the dichroic fog mentioned in the FDC with some fast films and stand development is a common problem these days. You could always test. You could also use longer development times with reduced agitation, only once every 3-5 minutes, or at half, thirds, or quarters of development time.
Divided developers are self-limiting by design. You only get to use what developing agent is carried over from bath A to bath B.
BTW, Vestal's divided D76 was an attempt to recall a formula published by Paul Farber. Vestal didn't recall if the sodium sulfite went in bath A or bath B and couldn't locate the magazine article, so he just split the required 100 grams evenly between baths. Vestal also recommends 'about 10 minutes' in bath B in his book The Craft of Photography. The 3-5 minute recommendation is from Anchell's Darkroom Cookbook. Because of the self-limiting character of divided developers, there may not be a lot of difference in results from lengthened time in the second bath.
Lee