Not certain that anyone here has specifically said they were trying for twins. Each of my Aus transfers was a double to increase the chances of even one taking. My own egg transfer that resulted in a pregnancy was a double that resulted in a single pregnancy.
In CT we did a double transfer as we only had two viable embryos (none spare) and we didn't want to transfer one, not have it take and then have to turn around and go back to use the second embryo. We were happy to achieve a viable pregnancy and look at the fact that the result was twins as a side effect rather than our goal.
Each person/couple has difference circumstances, preferences and background that will drive their decisions and the single or double transfer choice should be theirs to make with the appropriate advice.