The pair will be in charge for the Magpies' League Two match at Rotherham on Saturday following the departure of Ian McParland on Monday.
Director of football Sven-Goran Eriksson will continue his search for a full-time replacement for McParland.
Kevan, a former Magpies midfielder, has previous experience of taking temporary charge of a club, having done so during his time at Stoke and Burnley.
Johnson retired from playing at the end of last season and has been working as County's youth-team coach.
Source: Team_Talk