TheRedHead:If you follow all the procedures associated with the method (4 inch numbers indicating MOD, bottles hooked in proper order, tracing the regulator back to the bottle, checking depth and having all this confirmed by the team captain), it is the safest method. You have 2 people verifying the gas switch. But all the pieces are necessary in order for it to work.
This is the text book answer,but rarely find this is how it works. Sometimes people will have different algorithms for deco so they may be seperated by a stop or two. Sometimes the cave area for me can be so tight that we can't get two people together in the same area;plus other constraints. It is good to follow these procedures,but have something worked out that allows you to identify gas quickly,and accurately without a dive buddy around is important,because the dive buddy should be more of a redundancy than the primary means.