http://home.earthlink.net/~gamba2/images/macIIfxsch.GIF
Each battery is 3.6V. I'm not exactly sure from just glancing why they designed it this way, but it appears that this voltage is not high enough to turn on the power supply, so a second battery is added in series to bring it to 7.2V. D6 and D5 look to simply be there for a ~0.7V drop each, bringing 7.2V down to 5.8V.
Why they made it this way rather than scratch their heads for a while and reduce it to one battery is BAFFLING. At least the IIfx has battery holders, unlike the Mac II, which had soldered-on batteries!