No, you don't need the additional layer of plastic over the kraft paper for a vapor barrier. Plastic is more efficient than paper but shouldn't matter in your basement and especially where you have allowed 8" for space for the drains that is going to cause humidity behind the wall anyway. You may actually not want a tight or any vapor barrier at all since you have the french drain cavity behind the wall creating humidity since it all needs a place to go. (just reread your post and found you said septic drain which is a closed pipe and not really an open french drain as I understood but that doesn't change my answer)
If you had no french drain (or septic drain) behind the stud wall, then I don't think it would be a big problem (but still not necessary) to put plastic against the paper cause if they are touching then they shouldn't trap any moisture between them except if there was an air cavity between the two vapor barriers.
Sounds like your basement will stay warm and toasty with minimal cost for heating as it is. This may cut down on your heating bill if you have been heating it and won't take much more if you haven't. Good luck.