Jason,
I dove with a 19cuft back mounted for a while. I never worried about an SPG for it, because I would have to either route it forward and possibly get in the way of other things, or just not be able to see it. Instead of an SPG, I made sure to hook up my primary reg setup to the bottle before I dove, to check the pressure of the bottle. The fact of the matter, in my mind at least, was that if I ever went to the pony, it was a serious mistake, and I'm now working on my bonus life. In other words... I didn't care how much gas was left in the tank as long as it was full when I started the dive. It was there to get me out of the water, and I don't need to know the pressure to know if it's going to get me out of the water... I'll find out if it does or it doesn't.
I tried a number of different methods to back mount the bottle. I was never comfortable with it mounted right-side up, because there was absolutely no chance I'd be able to reach the valve. Since I was using my old cheap first and second stage on it, I didn't want it to freeflow at depth or anything, so I wanted to keep it charged but off. So I opted for it to be rigged upside-down, where I could easily reach the valve and I could have the second stage bungeed to the tank, with the regulator right by the valve [turn it on, grab the reg, pull it to me].
The problem however was I never found a really decent mounting method. I tried a number of systems, and finally settled on the Q-Connect by Kohr, which in it's own right was great, but it still wasn't ideal. You'll have to weigh the difference between the convenience of a quick connect method that attaches to your tank band, or a more permanent method that attaches to your tank itself. The attached to the tank itself makes it tough for travelling, and tough for changing between dives. Making it attach to the band makes it a little more floppy.
When I changed over to a BP & Wings, Instead of one center strap, I ended up with two straps. Trying to choose which strap to attach it to was difficult. Neither would allow me to get a good balance point on the bottle to mount it upside down. I ended up needing to mount it right side up to limit the flop. However that took me back to square one with me wanting to dive it with the valve off. So that's when I went to a stage rigging... and then a larger bottle as I calculated ascent rates and stops that I wanted to do for safety, making sure I didn't have to compromise my safety with slow ascents and longer safety stops for something simple as not a large enough bailout bottle.
Now with the larger bottle, and the SPG in an accessible location, the SPG is a little more useful in that I can start my ascent as soon as I have to switch to that bottle, and then do the stops and ascent rates that I would normally do, and leave 15 feet with a nice tank pressure like 500 psi.
I definately wouldn't concern yourself with two regs. The situation where both of you have problems and _both_ have to use one small bottle to get up is extremely unlikely. I would rather suggest that you fashion a method where one second stage would work for your use, and also be usable by an OOA buddy. A 40" hose should work for that. I actually really like the stage rigged bottles for this sort of situation. You can put your buddy on your alternate source off your primary tank, get them relaxed. Then turn on the bailout bottle, clip it off to them, and then you both can ascend without being tethered together and running a risk of getting in each others way.
As far as trim goes...This comes down to what you have for a BC. With a jacket BC I needed a 2 lb trim weight with my 19. When I switched to my BP and wings and the stage bottle setup, I strapped that 2lb weight to my backplate. However I found after I removed the gossets on my dive rite wing, there wasn't anything aside of water pressure compressing the air in my wing. This made it so when I shifted my body to be trim in the water, air shifted over to my left side and compensated for the extra weight on that side. It's nice like that, because if I use any of the gas in that bottle, the trim-compensating air will just shift and keep me trim. If I need to hand the bottle off to my buddy; I don't have trim issues the other direction by a weight that doesn't have a balance on the other side. I've never actually dove a back inflate BC, so I can't confirm if a back inflate BC has the same shifting properties as an un-bungied wing does; but it might.
As to everyone else in this thread. Not everyone has the luxury of having good regular buddies. And even those that do still have separate lives from their buddies and won't always be on the same vacation. People often say "just don't dive", but you can do one of two things... deal with the situation your in, get out, dive, get experience and improve your own skills, or sit on your ass talking about diving on the computer because you don't have anyone you can buddy with. I personally have a group of buddies that I will use for the more challenging dives. I then have a group of people that I will dive more simple stuff with. When I dive with a relatively new person, I prefer to have myself completely self-sufficent, so that
I can focus on taking care of them and making sure they are safe, and they don't have to be destracted by making sure I'm ok... they can focus on getting themselves more comfortable under the water. Yes, I will take a bailout bottle on any dive over 60 feet with someone that I've not dove with or someone that I know isn't necessarily comfortable at that depth. And besides, the way I rig my bailout, and the size that the bailout is, taking it in the water as a bailout also gives me the side benefit of practicing the same setup I would have if I was doing staged deco diving.
It's really a matter of getting all the pros and cons, and deciding which pros you like enough to compensate for which cons.
