I may be wrong but this is what I think: What did/do you see? 你看到什么了？ 到 is added after the verb to show it was successful and 了 for past tense. It's not something which is still happening, so no 在. 你在看什么？ What are you looking at? This is happening (在 and no了) and hasn't been successful - you haven't seen what you are looking at as you are still looking (no 到). ===== And the same with "who": 你看到谁了？ Who did/do you see? Who are you looking at? 你在看谁？

what do you see?你看到了什么？ what did you say?你看到什么了？ in Chinese“了”represents a kind of tense.it usually represents the past tense.but it has different grammatical structures in Chinese if placed different .many Chinese usually use it but can't clear its other grammatical structures except the past tense. translating these two sentenses into Chinese .it hardly to look differences.

What do you see? = 你 看到 什么? Whom do you see? = 你 看到 谁？ What are you looking at? = 你 看 什么？ Whom are you looking at? = 你 看 谁？

i think these two sentence are correct.”你在看到什么？“ cancel ”在“ or "到”。both can't exist in one sentences.