This is a fascinating scenario that could go in different ways and is challenging to predict.
But I would have to err on the side of believing if you are getting any positive effect from taking fent after bupe, then you are very likely to experience severe negative effects if you take bupe 36 hrs after fent.
It is possible that the negative effects will be mild. It is possible depending on tolerance and recently habituated activity that the bupe will only sober you up. It is also extremely possible that you will be in hell...
If you perform this research activity please be sure to report back and share your findings. Be safe.