My company policy is that you can use a single ETOPS alternate if you need to, but it's preferred to not. If it's a case of nothing else is legal at dispatch and you can make it work with one, then go.
Personally, I don't have any issues using one alternate. The ETP issue isn't a problem, since it is to determine which alternate you would go to if you have multiples. In the case of one, you're always going there (per the plan). If the weather drops, or the airport closes, make a new plan. if you can't continue on safely... go back. And if the catastrophic happens, i'm sending the airplane to the closest piece of concrete (or gravel if need be), whether its a closed AD, a taxiway, or fogged in.
I've never had to use a single ETOPS altn, and more often then not it will add more ETOPS fuel then using 2 or more. Plus ou pilos aren't trained on single ETOPs ops, and it just leads to a lot of questions I try to avoid.
Later