How to get others to understand that my first name consists of two words?

I would make a point of signing my emails Abcd Efghjkl. If they don't catch on after recieving your reply, add a friendly PS just as you have been doing. It's your name and you are completely justified in wanting it said properly.

You could modify your P.S. slightly, to make it easier for native English speakers to relate to. For example,

P.S. My name is spelled as two words that go together to make one name, similar to "Mary Ann" or "John Paul".

The simplest solution is to add a hyphen between the two words: John-Paul. The hyphen indicates that your first name (or the meaning of the word in general) comes from the combined words, not from separate components. Modifying your name (yourself, your identity, etc) might be a wise choice to adapt to the environment. This will save you a lot of time and energy to educate people, not in email context, but also in paper.

However, if you really want to use the blank space instead of hyphen, in email context I would say that adding the PS automatically is a little bit redundancy for familiar contacts. I suggest you to have a text expanding program to mitigate the typing time.