For really big ships, two or more power units may make sense, for redundancy sake due to battle damage, and to reduce length of power cabling. (If your power is next to the drive, in the back of the ship, and something cuts the main power cable anywhere, anything forward of the cut in the line is depowered). . .you might have either loads of lines, or multiple power plants.
But that would be on a rather large ship, unlikely to be one PCs would be running around in.
Usually, space is a premium, due to the fact you need to fit everything inside something. . .you want to have all your critical systems inside the armor as one for instance. So usually design consists of paring back things until you end up with less than you wanted crammed tightly into too little space. . .there's a reason submariners tend to be skinny and/or short. This tends to stress efficiency, rather than clever trickery. . .a large naval ship might be inefficiently built with two power plants fore and aft, but can afford it as the benefit in combat efficiency and redundancy in being able to shut one plant down (or have the power runs to it cut by combat damage).
You're also likely to see naval ships built to run at say 80% of their powerplant(s), to leave 20% (or more) for "emergency power" or damage.
That said, Defendi's comment about brownouts is very true of high tech gear, and likely any ship with combat control would have automatic breakers. . and a hierarchy of systems. . . so if the ship loses 50% of it's power, rather than run at 50% on all systems, half the systems on the ship shut off completely. . .so if damage drops power, rather than everything running at 1/2, half the systems shut off. Though if some systems do have variable power levels they might just drop to a lower level.
Which I suspect is the root question of this thread. . .which systems in a ship can be turned down, rather than turned off, reducing their power needs?
