So, I have a chord progession that goes like this: C#7#9 to F#13 or V7#9 to I13. What scale should I play over the 7#9 chord? I would assume use Mixolydian for the dominant 13 chord, but I'm stumped with the 7#9 chord. Thanks for any help from anyone.
Altered dominant chords such as #9 are cool because most anything will work over them. Interestingly, the Mixolydian scale would not be the most common scale to play. Most often, the base will be the minor pentatonic/blues with some extra tones thrown in, such as the #5 and the b9.

What's cool but hard about jazz is that, when you have two chords that aren't really related, the soloist often approaches each chord as an island. You think, "What goes over C#7#9?" and play it, then, "What goes over F#13?" and play that. With experience, you'll find ways to link the two islands with interesting and unusual licks.