Head Coach Position

Theoretically, what’s stopping the Texans or any NFL team really from hiring 2 Head coaches? Like if the Texans wanted to fire Bobby and then Go after Johnson from the Lions as their “Offense head coach” and let him hire his coordinators etc. we all know Ben Johnson will likely be hired as a head coach somewhere and wouldn’t even entertain an offensive coordinator position so why can’t the Texans do it and keep Demeco Ryans as their Defensive Head Coach?