Does the second account take up another X Gb of hard drive space or does it recognise that you already have the game? OMG I would have to grind all the engineers again... maybe I buy SC instead lol
The most simple way, is to change login in the Launcher, they use the same game installation files. And works basically the same if you launcher downloaded directly from FDev or Launcher from Steam.
And for more advanced users, you can automate this process of switching commanders, but you will always have to exit the game back to the desktop/launcher.
So no, you do not have to have two or more copies installed.