Generally when I want to open up a buffer in a window split, I use :sp or
:vsp and type out the filename. It may be a bit more convenient to use
window commands to create the splits.
To open a horizontal split:
Ctrl-w s
To open a vertical split:
Ctrl-w v
With the split made, you can use Ctrl-o/Ctrl-i,
BufExplorer, or whatever way
you like to navigate between buffers.
h/t Ryan Messner