When I first got Sammi I was worried Bailey would hate her, but we made it a gradual process getting them to do things together.
For about two months whenever I did something with one cat, I would do the same thing with the other. This way they didn't think one was getting more attention than the other.
My advice would be to get a feather on a wand toy like Da Bird and encourage them to play together.
Whenever you groom one, make sure to groom the other too. Make feeding times the same, but in seperate bowls. Give them each their own litter box, plus maybe an extra one.
But mostly just encourage them to play together, and do things together.
That is what worked for me.