呢句兩個that that 孖住用，啱唔啱？

文法你冇錯ge，第一個 that 係 pronoun，而第二個 that 係後面個 clause 嘅一部分，修飾第一個 that。只不過咁樣好唔自然，一般唔會咁孖住用。


建議直接地寫成：
What is under the desk?


2014-02-25 18:42:18 補充：
或 What is that thing under the desk?

It is ok.But not good.

What is under the desk?/What things is under the desk?
Your is ok.But not good.

What is that which is under the desk? ~ rather formal
What is under the desk? ~ much neater.

It's really a question of style, so be sure to take into account the tone and the level of language you are aiming for.