get in on sth....點解 in 同on 可以一齊？

Prepostion is word or group of words (eg in, from, to, out of, on ) which is often placed before a noun to indicate place, direction,(as in 1 ), source, method,(as in 2 ), etc. It happens (1),(2)are made up of group of words used as containing two prep.adjacent to each other as the meaning requires.
eg:-Get in (1) on (2) a bike into the garage building.

不行
因為in和on皆為介係詞
介係詞 (prep)加 名詞 動名詞 人 代 受