When to use which echelon?
Cycling is both cooperation and competition.
In a team time trial there should be full cooperation.
In a race, it is all about winning, so you are in competition with the others. But sometimes you are working together for some time, for example in a break-away, after which all will pursue their own wins. How much energy you want to spend in the joint venture depends on your objectives and temperament.
A single echelon is the most appropriate to be able to make big differences in work. The riders all ride at the same speed all the time, but take shorter or longer turns.
In a double echelon anyone does about the same amount of work. That means that the weaker riders must put in relatively more effort than the stronger.
For some, the fun of a double echelon is larger than a single echelon: everything runs like clockwork. Beautiful!
Team time trial
For a team time trial, a double echelon is sometimes used in the first part of a long time trial. Optionally, if the road is too narrow for the whole group.
But the best result you achieve with a single echelon. You’ll make sure that everyone is completely empty when (s)he stops. A few are expended on the way. If done properly, the rider that is about to be expended will take his (or her) final lead, and give it all, making sure the pace doesn’t drop. Once that pace becomes unsustainable, the lead is handed off, and this rider lets go of the echelon. The rider should never be dropped because he fails to join the echelon because there is an acceleration. That means that this rider has not expended all his/her energy, which would be a waste of resources. The remaining riders divide the work so that everyone in the team is completely empty at the finish line.
If you are already recovered halfway through the echelon, you are not putting in enough effort, and you should take longer turns. If you are in the second position (just before you take the lead) and you are still in the red, you should skip a turn or take shorter turns.
In an escape, a double echelon is very suitable. You want to do as much work as the others, but certainly no more, and the others won’t allow you to do less.
During an escape, you will often see both single and double echelons.
When cross winds are expected in a race, you often see multiple echelons.