I'm not trained for side mount. I've got a stage kit, and I've dove with a 130 cf HP steel tank on my back & an 85 cf steel tank 'slung' clipped to my left BCD D-rings. Later got a 30 cf pony bottle instead, and recently picked up a 3 cf Spare Air I haven't tried yet.
Our local quarry requires solo divers be solo cert.d & I believe have a redundant air source, and that's where I've dove like this.
I considered side-mount, mainly in case I go back to Bonaire, but the hassle of messing with the more elaborate (& bulkier, with 2 tanks) rig has to be weighed against just getting out & hooking up another tank.
Some travel destinations have at least a limited supply of bigger tanks. Some examples:
1.) Rainbow Reef Dive Center, Key Largo, in 2013, had 2 100 cf AL tanks, I believe, which were what I used with them.
2.) Sun Dancer 2 (now called Belize Aggressor IV) - had me a 100 cf AL tank.
3.) Jupiter Dive Center, Jupiter, FL - had 120 cf steel tanks.
4.) Olympus Dive Center, Morehead City, NC - had 120 cf steel tanks.
5.) For whatever reasons, for a planned trip (Lord willing & providing) to Southern California in August on a limited load Southern Channel Islands live-aboard, the affiliated dive operation had at largest 95 cf tanks, and from what I understand really big (e.g.: 120 & 130 cf steel tanks) for rent can be hard to find out there. Very strange to my mind; California divers often wear dry suits, some dive solo, conditions are colder than Caribbean diving & entanglement (in kelp) a risk...seems like really big steel tanks would be a nice differentiator for an op. offering such.
When I shop a dive destination operator, all other things being about equal, if you've got steel 120 cf tanks & your competitor is 100 cf & less, I'll probably book with you.
Richard.