Johnson, 18, has been on loan with Championship club Doncaster, while Mullally turned out for Batley on dual-registration last weekend.
Anderson said: “Mulls will be available for Batley again this week after playing 40 minutes or so for them last week. John Kear is very happy with how he is going and he will be fresh for the Easter weekend with us.
“Josh Johnson will also be back from his loan spell with Doncaster come the Easter period so he will be another option for us.
“It was a coincidence that Doncaster played Batley last week too so John Kear now knows that Josh Johnson is a good player so he may use him in the duel-registration system in the future.
“Both lads needed to play games and they have both more than equipped themselves in the two games so far.”
Mullally joined the Giants in the close season from Widnes, having made eight Super League appearances for the Vikings in 2012.