This is the issue reported in rspec#1391 - expecting like

    expect { puts "foobar" }
      .to output.to_stdout(/foo/)
      .and output.to_stdout(/bar/)

fails, because the two matchers get nested, and inner matcher catches
the stream write, hiding it from the outer one.

For the simple matchers, the solution is straightforward (and @pirj
supplied it in the issue thread). Pass the output along to the original
stream after storing it, unless the original stream is the _real_
original stream.

Sadly, it doesn't quite work on StdErrSplitter, because of the existing
(but possibly non-impactful?) bug around restoring the original stream
during the ensure block.