Scroll two buffers in unison

[Harry Putnam]

Pretty sure I've been able to scroll two buffers in split frame in
unison somehow.  I don't mean in ediff mode.

Calling C-h b and ferriting out all instances of `scroll' seems like
it should lead to such a command unless it isn't bound.  Not sure how
to search unbound commands unless C-h w displays them but I didn't
find it there either.

I went through M-x apropos <RET> scroll <RET> which produces a
massive list but still didn't notice a likely hit.