When training for the marathon, I would suggest striving for as many miles as possible in singles. If you can handle your planned 80-86 mpw all in singles, that would most likely lead to better results than doing the same 80-86 mpw in doubles. Of course, there are many variables that go into this but I’d suggest starting by trying to do singles and only add doubles if you are having trouble handling the workload with singles or if you get to that 80-86 range with singles and feel like adding more to those runs wouldn’t be reasonable but still feel like you could add a bit more by doing a few doubles a week. If you do add doubles, consider them secondary workouts and don’t let them take away from your primary workouts. Also, if you do have to add doubles, remember that it may be possible to transition back toward singles once your body adjusts to the additional workload if desired.
Another consideration would be to stick with singles in base training but transition to doubles when you start doing hard workouts. Using doubles then can help reduce total training stress and leave you more prepared for the focus of that phase of training.
If you don’t mind a question, 80-86 mpw for a 3:15 marathon? While I don’t know all there is to know about your running, this just strikes me as a lot of work for that kind of goal. I would expect someone who is running that kind of volume to be in the sub-3 range. Have you looked over the rest of your training and considered the possibility that maybe other aspects of your training could be beefed up a bit to balance out that volume and lead to better results? I know I seem to be constantly using total training volume as an example of how most people overlook key aspects of training to focus too much on only one variable, usually the long run, but it seems like you may be in the inverse situation of focusing too much on total volume at the cost of other aspects of training.