I have been learning how to make GUIs with Tkinter and a question has occurred to me. As I'm testing the program, I often end up building the code while an instance of it already exists in the background. Are these two independent of each other in terms of performing their functions? I've always read that when I create the instance of the Tk() and then run its mainloop(), that is what takes care of everything. Can I have two or more loops running if each pertains to a different Tk() instance?
Each instance of Tk is separate from any other instance of Tk. It gets its own copy of an underlying tcl interpreter. Two instance of Tk in the same process means you have two active tcl interpreters. 
As a general rule you should only ever have exactly one instance of Tk, but I suppose if you fully understand how they work, it should be possible to have two or more. 
I think this falls into the category of things you shouldn't do until you understand why you shouldn't do them. And once you understand, you won't want to do it.
Is each instance of Tk() class running independent of each other?