What happened duiring the first introduction? Did you introduce them in a neutral territory?
Are they both spayed?
If one or both are aggressive towards each other, you might try the stressing method - take them on a car ride first.
I'm not sure what you know about bonding so I won't overload you with stuff you may already know,
Just let us know what you've done so far and what you know about the bonding process, and then we can fill in the blanks.