I dont understand my cat help me?

Ok so i just got a new female(Mittens) cat 7months 2day's ago and my old cat (Chubbz)Male also 7months old. Chubbz isn't getting along with her he keep growling and hissing at her and she is meowing and puuuring . How can i make this stop. She is crying because he wont play with her she just goes under then bed. Chubbz keep's following her ever time she moves but when she turns around her hissing and try's to hit her :( . I tried butting them in septate room's but Mittens keep's crying to come out. they are using the same litter even tho i have 2 ? what does this mean i want them to be friends its making me sad when Mittens is crying to play