I'm not sure if you've already read it, but Jon Grey has written a fairly long series called "Jen and Ellen." The characters become lovers/girlfriends pretty early on, but with both women wanting to be the dominant partner (and also having a more submissive side) there's a lot of juicy conflict and tension. And several other women to act as third wheels, too!
If anyone's interested, the series can be found here, under "Jon Grey":
The HUGE list of female rivalry's erotic stories
There's both titfighting and sexfighting in it, but the main characters/lovers fight with their tits only once, in the beginning. All the subsequent competitions between them are sexfights.
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