fwiw, I've not used 744 or USBPre 2, however, I've regularly run MixPre-D (12V) and PMD661 (5V) simultaneously from the same battery (a dual voltage Tekkeon MP 3450/R2 50Wh Li-Polymer) and not experienced any issues circulating between devices.
Past tense because I don't run the Tekkeon MP3450 any more, it failed prematurely after only one set at my last show, then it wouldn't take a charge. It appears Tekkeon no longer sells these and it is out of warranty so I took it apart and found one bad cell. Unfortunately, the exact cells they used in them are no longer available and the closest replacements wouldn't fit the case. This was the second Tekkeon failure I've had, first one happened just outside of warranty when they were still selling them and they grandfathered it sending me a replacement.
Knowing the age of the Tekkeon I carried a Sony NiCad NP1 battery with a battery cup as a backup so little was lost. Just this week picked up a pair of 14.4V 5.1Ah(73Wh) Li-Ion NP batteries and charger for the MixPre-D and will run a small USB battery for the PMD661. The NP and USB batteries both fit in the rig bag nicely and I'll carry the second Li-Ion NP and USB battery in an accessory bag with eneloop "AA"s in both the devices for a quick switch backup or battery swap if needed.
I enjoyed the dual voltage Tekkeon and while it worked it made a nice solution but now in 2016 have decided to skip dual voltage batteries.