I know this has been an issue for a while and I found a lot of discussion about it, however I didn't get which would be finally a way to get it done: pipe both, stdout and stderr. In bash, this would be simply:
cmd 2>&1 | cmd2
That syntax works in fish too. A demo:
$ function cmd1
echo "this is stdout"
echo "this is stderr" >&2
end
$ function cmd2
rev
end
$ cmd1 | cmd2
this is stderr
tuodts si siht
$ cmd1 &| cmd2
rredts si siht
tuodts si siht
Docs: https://fishshell.com/docs/current/index.html#redirects