Of course your mileage may vary and a good chunk of the time this will work out fine, but IMO this is a terrible idea and you should reconsider. As has been told in numerous threads all over FT, WN's current operations often leave something to be desired. If your initial PHX-ATL flight is at all delayed, or canceled, or has to wait for a gate, or has some other minor unforeseen issue, you are going to miss your flight and lose your DL ticket. Yes, you will probably be ok, but there is a lot of risk with booking this as you've outlined.

EDIT: I just realized this is going from DL to WN, which I think is less risky overall (better DL ops than WN), but still way to risky for my taste. 59 minutes for a terminal change to another domestic carrier is still risking losing your entire second ticket as a no-show.