Ican't see them fighting each other. They have too much in common. They both likes guns and poetry, they hate Cerberus and many other things. I think they will get along nicely, just like in Citadel DLC
What if they both wanted Shepard? Think about it. Do you really think Ashley will think Jack, someone who's committed multiple crimes, would be good enough for Shepard and do you really think that her attitude won't get under Jack's really thin skin?
There are a few possible situations like that in ME1 (Liara), or ME3 (Jack, Miranda, Tali) and Ashley's answer for them where always similar: I'm not happy, but if it's what you (Shepard) wants, so be it. This scenario could possibly work with Miranda, but in Jack's case Ashley's protective elder sisters wibe will probably kick in (just like with Tali). I know that having two hot girls fighting for you could be entertaining, but I can't imagine it between those two. Especially not with ME3 Jack.
*Walks in* Me: Ladies, there is enough of me for the both of you. *Ash and Jack put their guns down and look at me with a smirk on both of their faces* Me: That's it, come to daddy. *Gets the crap kicked out of him*
THAT's what it's called!! Geez, I've been wracking my brain for an hour trying to figure out the proper word to type in Google and I wasn't able to find a good reference pic in the process, so I just wung it. Thanks, glad you like it!
They... are... GORGEOUS!!!!!! Maiden, I don't know how you do it but... WOW! You outdid yourself with this one! I'm in love! Will you look at that Jack?!!! Mmmm! Superhawt sexy suit, bitchy expression, YEAY!