Yeah...that's what I thought too. Individually, both calls work. ArmyBrain:GiveStorage('MASS', 10000) will correctly augment mass storage by 10000 and ArmyBrain:GiveStorage('ENERGY', 10000) will correctly augment energy storage by 10000.
Try them both together though
ArmyBrain:GiveStorage('ENERGY', 10000)
ArmyBrain:GiveStorage('MASS', 10000)
and only the second function call will remain in effect. Throw in a WaitSeconds(5) in between the calls and you'll see that the second call replaces the first. I get a +10000 energy storage boost, but as soon as the second call is made, I lose the energy storage boost and I get a +10000 mass storage boost instead.
So as best I can tell, I can have a mass storage boost or an energy storage boost but not both.
By the way, I already came up with my workaround. I created a custom unit with the mass and energy storage values that I needed and spawned it off the map where, hopefully, it will not be seen or disturbed.