As far as I know you can't, but I would really love to have a few open when I'm working on multiple projects at once, or when I'm comparing different source codes.
SED has tabs available, but not everyone likes tabs. Having multiple windows of SED in the task bar could be handy for swapping between different scripts and even having two scripts side by side for comparison. An alternative is to use Wordpad or something, but then again, code isn't colored so it can be trickier.
"You level up the fastest and easiest if you do things at your own level and no higher or lower" - useful tip
My 2D game - release on Jun 13th; My tutorials