Mujeeb has been retained by the Heat following his return of 12 wickets in the 2018-19 and an impressive economy rate of 6.04 which was the best of the competition.
Zahir, who is currently playing for Jamaica Tallawahs in the CPL and averages under 20 in T20s, will join him to share some of the spin-bowling duties during the season. Under new BBL rules, teams can sign up for six overseas players for the season - up from the previous four - although the limit in an XI remains two.
"This is a very exciting thing for me, especially to be joining the team where my good friend Mujeeb played last year," Zahir said. "He told me great things about Brisbane Heat, so I am looking forward to this challenge so much."
Heat's head coach Darren Lehmann said: "The new BBL contracting rules this season gave us some flexibility to work with them on any potential commitments around our tournament and we are very happy to have them joining us
"I think the arrival of Afghanistan in international cricket has been one of the best things to happen to the game in recent years, and it is a real joy to see players like Mujeeb and Zahir grow in stature for their country and show their skills around the world.
The Heat narrowly missed out on a place in the knockouts last season after a strong late run in the group stage. Brendon McCullum ended his long association with the team at the end of that campaign.