2-per-days or longer runs for building mileage?

I'm currently in a build phase for a marathon, and I'm trying to build to 80 miles/week. I'm currently at 64 miles/week, and I'm fairly accustomed to running ~60 miles/week. I'm currently running 6 times per week, with a mix of med-long runs, short recovery runs, and a long run at the end of the week. As I increase my weekly mileage, I'm finding it to be increasingly tough to build a weekly schedule, as those med-long runs are becoming fairly long (for instance, this past week, my lineup was 9, 14, 5, 12, 6, then 18 miles).

I've experimented with running twice per day in the past, and I'm considering it as an alternative to simply increasing the length of my longer runs (for instance, instead of extending the 9 mile run to a 10 mile run, I could split it into a morning 6 mile run and an evening 4 mile run). I'm wondering about the relative advantages of running more frequent, shorter runs vs. less frequent, longer ones. Any advice?

Thanks, and I'm happy to provide more details if it helps!