Split the front and rear final tracks to mono
Duplicate those four mono tracks (four will be used for LT, the other four for RT)
Rename all eight mono tracks using their proper speaker position (LF, RF, LB, RB)
Following the encoding parameters given by Leonard Feldman in 'Four Channel Sound':
For LT
Keep LF the same
Reduce RF by about 10.5dB ((log(0.3) * 20))
Keep LB the same
Reduce RB by about 6dB ((log(0.5) * 20) and invert it
Mix these four tracks down to LT
For RT
Keep RF the same
Reduce LF by about 10.5dB ((log(0.3) * 20))
Keep RB the same
Reduce LB by about 6dB ((log(0.5) * 20) and invert it
Mix these four tracks down to RT
Rejoin to stereo