If you follow coggan he agrees 2x20 in sweetspot zone is not effective. 2x20s at 95%+ is the staple.
I wouldn’t say that it isn’t necessarily effective, but if you’re solidly in the “sweetspot”, you can certainly do a lot more than 40 min of training, and there is no need to break it into repeats.
(Note that just because you can doesn’t mean that you should, but that question can only be answered on an individual basis and within the context of the entire training plan.)