Usually : ring finger for the low note; index and middle for the high
occasionally : little finger for the low note; ring, index and middle to have fun on the high note.
also occasionally : thumb for low note, all 3 'big' fingers together for high, in a crab-like pincer motion
another thought would be - why not combine slap and finger style? with the right bass, setup and maybe a bit of compression, slap doesn't have to be loads louder than finger style...