Help fixing issues 4182 and 4691

[Kevin Barry]

Hi Developers,

I'm struggling with trying to fix these two issues so I'm asking for
help from someone with a better knowledge of LilyPond internals.

I have spent a couple of weeks looking at the code and trying different
solutions (in both C++ and Scheme), so I think I have some understanding
of the problem, but I have no idea about how/where to solve it and would
like some guidance. I won't go into details about the nature of the bugs
but they are both related to cross-staff beams.

I know everyone is busy, but I think it would be worth the time to help
get two regressions fixed (and maybe I'll learn something from it that
means I won't need help next time). Feel free to reply on or off list.
I'll be very grateful!

Kevin