Little

Couldn't think of a better forum than this one to post it in ...
My boyfriend's cat just started her first heat cycle, while mine has been spayed already for more than a year. They've just now started to get along.
Today I was watching them play when my kitty started biting the other's neck, like a tomcat would do ... but kept her hindquarters away and to the side.
Is this safe? I can't tell whether they're fighting or whether my kitty is trying to alleviate her friend's distress somehow. Anybody have some advice about it? For now, I'm just keeping them separated.
BTW, kitty #2 is going to be spayed asap. I already decided (when I spayed mine) that even if it is expensive, it's worth it not to live with an unhappy and meowing cat.