First, valves are in visual site, blown o rings or other failures are immediately noticeable. Second, valve is able to be turned on/off examined, shut down in emergency quickly and easily. Third you can even connect/reconnect first stages in event of failure to see if it alleviates problem, while breathing from other gas supply. Fourth each tank has it's own first and second stage regulator- redundancy - a failure in one still leaves the other with plenty of gas. You also can even swap the bcd inflator hose in event of failure in one side. Fifth, Weight is better distributed for buoyancy and maintains trim in sm configuration. It also takes the pressure off the back for people with back issues on land. Sixth, if you use two 40 or 50 cu steel tanks you also have better weight distribution and the same volume (more if LPs are cave filled) of gas in a better package. Bottles can be carried one at a time to entry point and donned in water. Seventh, an out of air buddy need not be kept attached during ascent, as the donor bottle can be up clipped and passed, avoiding any issues of a panicked diver bolting to surface taking the donating diver along.