I am the least qualified to answer this question, but I am interested in FFMs because I think they look cool, so I have done a lot of thinking about them.
Just from doing my own research about typical hose routing and equipment setups among technical divers, it seems that you would have your FFM on a short hose connected to whichever post is most convenient (or modify the hose length to make sure it routs well on the left post).
Then the question of the backup regulator comes into play. You will be primarily using your FFM, and your backup regulator obviously will go on a long hose because you can't donate a FFM (anyone seen Sanctum? hahaha). So this means you are donating your backup (a slight drawback as you don't "know it works" because you aren't breathing off of it). In the event of a donation, you need to make sure three things happen in a normal setup. First, the donated reg should be on a long hose so you can move single file. Ok, simple, just keep your backup on a long hose and the FFM on the short hose. Second, you have to make sure that when you donate, the donated regulator is on the right post so that you are breathing off the left post and will notice if a roll-off occurs (since your buddy will have trouble letting you know if you are moving single file). Well, this is more complicated depending on the FFM you use, but if you aren't worried about a roll off (eg. not inside a restrictive cave) then leave it, otherwise work with your hose length and type (braided would work well here id imagine, as might a 90 degree swivel or something) to get the routing correct to keep the FFM on the left post. Third, and lastly, you have to be able to donate quickly.
This is what I have thought about most recently. Normally you donate the long hose, it comes right out of your mouth if you are using backmounted doubles, and goes right into your buddy's mouth. I would imagine when sidemounting or using a FFM, and the long hose is not in your mouth at the moment your buddy needs your long hose, you would have to unclip it from your right shoulder d-ring. This sounds bad to me. I would imagine that for sidemount or when using a FFM, you would want a "breakaway" device of some sort. But what kind? The stupid rubber things that go around the mouthpiece of a recreational rental octopus... F*** those things. If you look away for a millisecond the bastard sneaks out and tries to swim away. My octo falls out all the time, got me caught up in the only wreck in Cozumel once and ended up getting me and the two divers behind me (who I assumed were watching where everyone in front of us was going, but they weren't) completely lost inside the wreck.
You know what I think would work awesome for a breakaway? A hair tie. Those cheap rubberbands wrapped in some sort of sewn fabric. You give it a good muscle, and it will snap, but they don't break until you play with them for three months straight. I use those on my octo, and I have actually had a problem with my primary and needed to go to my octo once and the hair tie did its job, it broke when I needed a breath of air. I think if you take one and tie it onto a d-ring, and you boltsnap your reg onto the tied on hair tie instead of the d-ring, it works great. my octo has never come loose since ive been doing that (which hasnt been that much) but I see no reason why this wouldn't work for the long hose.
So with a FFM, in my imaginary world where I make a lot more money and have been diving for a lot longer and can afford tec classes and tec experience, the biggest differences to me seem to be making sure that your secondary (on the long hose) has a good, secure, but frangible in an emergency breakaway connection to your right d-ring, and getting the hose routing right for your specific FFM.
Adding more regulators in my mind seems rediculous. having three regs is just asking for it. I mean, you could put regs on all those unused ports in your first stages, but would it make you safer? No. A single secondary on a single primary is as good as it gets. Even in recreational diving, other than for air sharing, having an octo only increases your chance of equipment failure. Unless your primary has a problem with the exhale valves and floods uncontrollably, the housing shatters, or the mouthpiece falls off, having a backup secondary only adds to the number of failure points on the LP side of the first stage that could result in free flow and loss of your only tank (much more likely than the housing shattering, at least). In a situation with two tanks, you have redundancy by separating the two second stages between two first stages. Don't add in those extra failure points on either reg when you don't need to, because you already have the capability to save your buddy, deal with a mouthpiece falling off, or a second stage disintegrating completely. Adding more second stages just increases the chance that something will free flow and force you to stop using that tank/first stage completely.
