Is there a function to return buffer of the "other" window when split horizontally?

[Martin Nordholts]

Hello!

I fail to find a function that returns the "other" buffer when one has
split a window horizontally.

other-window seemed as a good candidate at first, but I can't make it
work as I wish.

How can I do this?

I need it for a function that changes a horizontally split window to a
vertically split one (to quickly make more rows availible on both
buffers when entering a Ruby debugger session.

Thank you!

- Martin Nordholts