A normal floppy is 1.44MB. If there is already 724KB on it and you try and put another 724KB on it there will not be enough room (724KB + 724KB = 1.448MB) to fit the second file. That is ignoring the losses due to formatting and hidden files.
Floppy disks are so cheap that I would have thought it is hardly worthwhile trying to squeeze so much information on to one disk. It is also good practice not to write too many times to a floppy, they can become unreliable with overuse.
As for the time aspect, you can't really be concerned about the minute or so taken to write a floppy disk, can you?