Concentrating: I do it.
I really don't know about D-76, but here's what I do with Xtol:
First, it's two component powders, so dividing up the powder is not for me. Boiling or even heating is out, too, because some chemical agents in the Xtol will go south above 30°C...
That being said I buy the 5-liter-pack and dissolve it double strength, i.e. in 2.5 liters of destilled water (just using an empty 5-liter canister of the distilled water for this). After letting it dissolve overnight and thoroughly stirring one last time, I fill it into smaller black bottles and top off with protective gas.
Usage has been over a year now and it's still good. I do test it before developing a film, by putting a drop of the stock onto the film ending and watching it turn black in the fixer...