Unit classes aren't split up in terms of degrees, so someone taking the same unit as you won't necessarily be doing the same (double) degree as you. For example, Engineering has a common first year, so people you meet in those classes will definitely be doing Eng, but they could be enrolled in many different double degree pairings (or they could be doing a single degree). You being the same classes as your friends for your Eng units is also dependent on if you all manage to get into the same class when you allocate preferences for or adjust your timetable.
Unfortunately, I can only answer your first question with confidence. To do with your device, I've vaguely heard people recommend a Windows OS for CS and Eng due to the software you're required to be able to use, though I'm not entirely sure. Hope others chime in!