Ashley and Tatianna storylines are connected. You will have different option to get tatianna before the end of season. Your relationship with Ashley could help you or make it impossible. it depends by to many variables. IF you're not interested in Ashley don't get her. There are another path about Tatianna that starts from 1.0.
I mean this game take track of many things so there will be different ways to get some events and there isn't a "right way". The scenario you create with your choices will give you different option (sometimes no option). anyway sex with Tatianna will not happen before 1.4/1.5. You have to give time for events to happen and get in position to exploit your position, whether it's blackmailing Connor, consoling Tatianna, making a couple exchange, or pushing Ashley to know her perversions.