Mandy is a great name. And 2 kittens ought to be friends in no time.
My 1st cat and my 2nd cat (both gone now) HATED each other.....for about 3 days. They were inseperable from then on.
On the other hand, it took a full year before Thumper (RB) was accepted into the threesome.
So it varies between individuals. Patience and try to let them work it out by themselves (supervise from a distance)