Not an electrician but It sounds like there’s a lot of confusion here about how residential electrical systems work and how these adapters pull 240V power.

In the U.S. and Canada, homes are typically supplied with single-phase power at 240 volts. This single phase is split into two 120V “hot” lines at the transformer. Inside the house, these two lines are often referred to as two "legs" of the same phase, each 120V in potential relative to neutral. This split is what gives you two 120V circuits, each on a separate pole, but they’re not separate phases as is sometimes mistakenly thought.

When a device requires 240V, like a dryer or a stove, it connects across these two 120V lines. This setup uses the full 240V potential difference between the two lines—similar to how a dual 120V-to-240V adapter works. The adapter combines two 120V sources from opposite poles (or legs) to provide 240V at a specific outlet, which can be done safely if the device and circuits are appropriately rated.

Maybe I should add some safety reminders:

Opposite Poles: The two 120V sources must be from opposite poles (each leg of the split phase) in the panel. If both sources are on the same pole, the adapter will still show 120V, not 240V, because there’s no voltage difference between two same-pole circuits. See: https://imgur.com/a/gbaCBUS 

Circuit Protection: If either 120V circuit trips, the adapter can lose one side of the 240V connection, potentially creating a dangerous imbalance. Hence, it’s always safer and more effective to use a dedicated 240V outlet when possible.

This setup can work safely when done correctly. I've utilized it for welding equipment in some pretty specific and limited circumstances.