At one time touchline coaching was officially prohibited at all levels of the game (but it never made any difference to parents watching their kids playing in the local Under-10s or to the managers of pub teams, yet alone in the professional game!).
These days, just ONE person is allowed to coach a team from the touchline. Quote:
"The technical area relates to matches played in stadiums with a designated
seated area for technical staff and substitutes as described below.
While the size and position of technical areas may differ between stadiums, the
following notes are issued for general guidance:
• the technical area extends 1 m (1 yd) on either side of the designated
seated area and extends forward up to a distance of 1 m (1 yd) from the
touch line
• it is recommended that markings are used to define this area
• the number of persons permitted to occupy the technical area is defined by
the competition rules
• the occupants of the technical area are identified before the beginning of
the match in accordance with the competition rules
• only one person at a time is authorised to convey tactical instructions from
the technical area
• the coach and other officials must remain within its confines except in
special circumstances, e.g. a physiotherapist or doctor entering the field of
play, with the referee’s permission, to assess an injured player
• the coach and other occupants of the technical area must behave in a
responsible manner"
Source:
http://www.thefa.com/~/media/files/pdf/the-fa-2013-14/fa-rules-2013-14/laws-of-the-game-2013-14.ashx