Since the OP is not a tech diver how does he know the single tank is not the optimum choice? That was probably why the question was posed here. It should be the responsibility of the instructor to inform him of that. There are applications when a single tank might be the best choice.
Please enlighten me as to which application, on a decompression dive, a single tank would be a better choice than doubles. I cannot think of one. And I believe this OP is like others I've talked to, he knows the vast majority are doing deco in doubles, but is for some reason looking for permission to do it in a single. Whether its a money issue, a physical ability issue, I don't know, but its been my experience that people who ask this question are looking for a way to try something they are not ready for. This person has probably already talked to an instructor and been told they need doubles. Someone then told them standards allow singles and thats why I think the question was asked. My post was intended to help inform him why some instructors require doubles for this class.