You are wrong about this. The structure of the characters is very explicit; one character per square. Meaning is assigned at that level and never below.
(As a matter of character etymology, you are right that 廿 and its variant form 卄 come from combining two 十. But that can't make it "two symbols".)
Because there is no similarity between those two things. "combine" and "-ing" have meaning independent of each other. (In linguistics this is recognized by calling both of them "morphemes".) This is not true of the graphical components that make up Chinese characters; the bottom level at which any meaning can be assigned is the entire character.
Characters have structural features that can be identified and named. But at that level of the writing system, meaning isn't present.
> This is not true of the graphical components that make up Chinese characters; the bottom level at which any meaning can be assigned is the entire character.
I don't speak Chinese myself, so I'm a bit unsuited for the discussion, but a Chinese friend told me that the characters are composed of base symbols. You can't necessary guess the meaning from that alone and sometimes they changed shape a lot over time, but that is true for other languages as well.
Maybe "-ing" is a less ideal example, because it suitability to be combined depends on word class (verb), and it actually changes the grammar, not the word itself. Maybe a better example is the "com-" in both "combine" and "compose". It does has some meaning on it's one, but what the actual combined word means is not necessarily directly inferable and has changed over time.
(As a matter of character etymology, you are right that 廿 and its variant form 卄 come from combining two 十. But that can't make it "two symbols".)