I totally back FiNGOLFiN's request. Either by opening multiple instances, or by opening multiple views next to each other.
My current reason is that I am adjusting multiple pictures and I want to be able to compare the white balance as DIRECTLY as possible. Not by opening an instance of some other program, but by using the very SAME program twice, or WITHIN the very same program.
It isn't that odd.
It is a pity that a program like Paint.net tries to be 'clever' by RESTRICTING me to use it multiple times. PLEASE do not bother, let ME decide what I need!
So I would appreciate if either that that restriction is lifted (the most simple AND versatile solution) or that multiple (side by side) views are built in.