this topic comes around quite frequently. You'll find something. Trust me.
It is unwise to attach your 'pony' or bailout bottle behind you to your single tank. You cannot reach the valve to turn it on or off. You cannot hand it off to someone else. In that location it is more likely to become entangled on something in such a way that you, unassisted, cannot untangle it.
Most bailout bottles don't have a lot of air to begin with. Bailout regs can have many of the same types of problems that your other regulators can. Therefore, charge your reg before getting into the water, then turn it off with pressure in the hose. If you ever need it, it takes seconds to turn it on - plus you know it will be full.
This is a better option for carrying a bailout bottle:
http://www.dive-rite.com/resource/stage/index.htm
Remember that you may need it,
or someone else may, but emergencies underwater have a distressing tendency to not occur as you planned.
Generally speaking it is wise to preserve as many options as possible while reducing potential liability to a minimum. Putting your pony on your back does not do that.
FWIW. YMMV.
Doc