A simple feature of almost all programming editors, having two files open side by side, vastly reduces the amount of short term mental juggling you need to do.
What's really nice is being able to split the window in which you're viewing a file, and keep part of it visible in one pane while using the other to browse or to read another part. (Or, similarly, being able to have two windows side by side on the same file.) When you have strongly interdependent sections of code which are far enough apart that they can't both fit in the same viewport, this is invaluable for quickly gaining a grasp of what's going on.