As noted in the Wrestling Observer Newsletter, the theme/tag line they are considering for the match is “Winner Take All.” That is not finalized so it could change before Raw on Monday. Also, that tagline does not mean that they will unify both titles because that is not the plan, at least not for the long-term.
The idea looks to be if Ronda Rousey or Charlotte Flair loses then the person who beats them gets their title. So, if Becky Lynch pins Flair then she would become the SmackDown Women’s Champion.
Rousey is not advertised for any shows following the post-WrestleMania Raw so it seems likely that she would lose her title and it would make sense for Lynch to be the person to pin her. That way, they can set up Lynch vs. Flair in a singles match at a future pay-per-view this year or for next year at WrestleMania 36 with the story being that Lynch never pinned Flair.
Dave Meltzer noted in the Observer that it was unlikely that one woman would walk out of WrestleMania with both belts because they want each brand to have its own champion.
Of course, if Rousey is convinced to stay a little bit longer then that opens up the possibility for her to retain her title and drop it at a future date. Whatever ends up happening, don’t look for one woman to talk out with two titles.