I did something with bash coproc

gistfile1.txt

drwilco@guari:~$ pstree -hp 23944
bash(23944)───grep(2832)─┬─grep(2836)
                         └─yes(2835)

(23944 is the interactive shell that I used to start test.sh)

test.sh

#!/bin/bash

coproc yes { exec yes; }

coproc grep { exec grep y; } <&${yes[0]}

exec <&${grep[0]}

exec grep y -