Originally posted by chazmox
Well you could make a case that you can only really use one operating system at a time. Even if you have two computers in front of you, your attention can only be on one at a time... Granted the time is shorter than in the sheet example but you really only have one conscious attention.
Yes, there could be some tasks that your waiting on one machine and then using the other and, therefore, using two OS copies at the same time, but those would most likely be associated with some other SW and not the OS...
Just a thought experiment - please don't argue back, just wanted to take another view on that argument.