How can i wire directly an Asian single 240 volt line with neutral instant hot water heater to my 240 twin line tap in the United States? Can i tie the two legs on the breaker into a single wire? I am not sure how the neutral would feed then. What is the best way?
Asian and US power work in a very different way. Both transformers are 240V. However Asia applies safety earthing on one end of the transformer's 240V winding. America pegs it in the middle. Philippines is a weird compromise because they have both systems and are in the middle of trying to harmonize on one.
The concept is that American power is safer since voltage is only 120V from earth.
Neutral is not earth/ground. Neutral is a normal current-flowing conductor like the hot(s), it is simply the one that uses the same transformer lug as earth. That makes it the "less dangerous" conductor. It is not earth and should not be connected to earth anywhere except one bonding location specified by Code.
Some appliances are built with weaker insulation on the conductor planned for neutral. Code doesn't allow them to do that, but they do it anyway. Not mentioning any subcontinents here.
Your water heater wants 240V across its two supply terminals. in Asia, one of them will be neutral, in the US neither will be. This use needs to be permitted in its labeling or instructions as approved by UL, CSA or other competent listing agency. CE is not a listing agency.
In a US installation of a five-continent 240V device, you will have no use for US neutral. You will need two hots, and this must come off a 2-pole 240V breaker.