IMHO TT is not required but certainly single T with disciplined casting. Not trying to be a smartalec, but do you understand what disciplined casting is? U might want to look at the thread on when to start using an ecollar. It shows the timing of my training with Rowdy. Swimby was delayed waiting on warmer weather. But look at the steps before and the time it took.
Rowdy would been about 10 months when ready to start swimby if it had not been for cold weather.