My first solo diving rig was a single tank setup (100cf) with a side mount, clip-on 32cf bottle. I hated having one bottle on the side, it was totally lop sided and in my opinion, putting your bottles on the side for recreational diving sucks. Yes, redundancy is awesome, but it shouldn't detract from your normal diving situation when you're coming up with plenty of air remaining.
My second solo diving rig was my favorite, but it was just too heavy for shore diving. Double HP80's with a manifold, Jetstream regs, OMS BPW and fancy quick disconnect system to change gas and bail out for my AGA full face mask, which I tended to wear more times then not. Again, the same 32cf pony bottle, only this time I hung it off the back of the backplate and had a really nice hose arrangement which wrapped around the left side and connected to a fancy QD A/B switch a buddy of mine made. So if you had a reg failure, or needed the backup air, you'd just switch the switch and the BC and AGA FFM would be on backup air. I ran an air-assisted inflator system which hooked into the A/B switch as well. So the backup gas would run the main breathing 2nd stage and a backup. If you had to bail out, there was another 2nd stage attached to the tank itself and you could literally take the BC off, unhook the tank, hug it and slowly float to the surface. It took me a while to get the weighting down for this rig, but it worked well. Sadly, I only did a few dives with it before an inner ear issue kept me dry for 6 years. Here are some pix when I went to sell it. Sadly it doesn't show the QD or hose arrangement from when I was diving with it. Wish I had some pix so I could remember how I built it! LOL
Getting back into diving right now and building a very similar rig, only based on dual 45's for weight reasons. I'm also using a smaller pony bottle and fewer hoses! I hope it works well, still debating on who's regs I'll use.
I'll always have 2 rigs anyway; a recreational/training rig which would be more set up like DIR and a more tech/solo rig with lots more redundancy.