Hard to believe something like the history of 5.25" drives is already lost in the mists of time.
The original 5.25" floppy drives, which I believe were the genesis of the 5.25" drive bay, were twice as high as what is now the norm:
http://www.old-computers.com/museum/com ... st=1&c=571
http://www.old-computers.com/museum/com ... st=1&c=274
http://www.old-computers.com/museum/com ... ?st=1&c=18
Thus, what we now consider the norm is called a half-height drive because it is half the height of the original, making the confusing answer to your question be: yes, two of these can fit in a regular 5.25" drive bay.
But, that is based on a "regular" bay being the original size of 5.25" drive bays.
The answer you really want is that a half-height drive is the current "normal" size, and only one will fit in a current "regular" slot.
cfitz