Mostly for grouping. In general I use SciTE for code, Notepad++ for everything else (I use zillions of text files).

Two separate applications give two distinct icons in taskbar, allowing me to switch fast between two groups (as opposed to figuring out which content tab in one editor has the right file).

Also, I like them both, and don't have a heart to drop using one of them :). BTW I do the same with browsers (and OSes, I keep "affair" with Ubuntu in VirtualBox).