Putting two of everything in one box requires almost the same space as putting the same components in two boxes. Yes, you can remove two pieces of thin metal (one side wall on each case) and save yourself 2% of the space.
If they are desktops, they can be stacked. If they are towers, they can be placed flush and adjacent. If they are servers, anyone could build a simple rack to hold them both.
Programmers needing two computers (working on one at a time) may find a mouse/keyboard/monitor switch is more space saving. That is: why use two mouses, two keyboards, and two monitors if space is at a premium and one of each is enough?