Reverting back to the original question, for a recreational diver, a 30CF pony bottle is perfectly appropriate. In an OOA situation it has plenty of air to perform an ascent with a safety stop from the deepest recreational depth.
As far as attaching it to you and a SINGLE CYLINDER, I like a bracket system that allows for attachment to the cylinder on your back with a seven foot hose as mentioned by Eric. The bracket allows for less shifting of the tank as you move and the longer hose allows for you and/or a buddy to use it in many different environments.
If you are set on the straps hanging from your D-Rings, that is also a good method and the smaller tank is less likely to leave you off balance when mounted out to one side. In this case, the hose clamps, D-rings and nylon straps with small caribeeners on both ends are the way to go.
As mentioned above, a Y valve does not account for being out of air - regardless of who should be watching whos' air supply. We train for these emergencies and take measures to prevent them because they have happened in the past.
My two cents anyway.