You can buy 24volt alternators with an internal regulater, (I bought one on e-bay when I converted my cat 12 grader to direct start) you may have to modify the mounting brackets & pulley. The charging wiring will also be just one wire. Good Luck
I don't know anyone who offers a ready to mount unit for the D6B but I'd look for one that has a heavy duty front bearing to handle those double belt loads....maybe from a D6C or similar and modify mounting to suit.
That's a good point O.M. the one I used on my conversion certainly would not handle a double belt set up !
If you are only driving the alternator with the dual belts, they don't have to be so tight you can play a tune on them, to turn the alternator. Just tight enough that you can't turn the pulley by hand.