> What happens if they boil at the same time?

Then they're boiled and you don't have to do anything. But the point is that they boiled in exactly half the time it would have taken using only one of the devices with the full amount of water.

> But if your focus is time, then boiling is not an issue. You can do whatever you want during heating.

If you're making a full meal, sure. If all you want to do is hard-boil a couple of eggs, then yeah you want to hurry it up.

If you really want speed, use your microwave as a third!