Is it grammatically correct to use two past continuous tenses in a single sentence?

Question

This question just suddenly somehow popped up in my mind.

Usually what I encountered in a sentence is in the pattern "past tense + past continuous"

E.g.:

I was doing homework when my mum came back.

I would like to know whether it is grammatically okay to use two past continuous tenses in one sentence?

E.g.:

I was laughing so hard when I was watching the video.

Answer 1

It's correct to use the past continuous 2 times in a single sentence to give background information:

1. It was pouring with rain and she was wondering what to do.
2. You weren’t listening to me when I was explaining the rules!
3. Who were they talking to when you were preparing the report?
4. I was laughing so hard when I was watching the video.

But we use the past continuous only once when something happened in the middle of something else (e.g.I was doing homework when my mum came back).

Answer 2

You can also use the past continuous for parallel actions: I was doing my homework when/while my mum was talking on the phone. These are two actions happening at the same time.

Answer 3

In accordance to the "Timeline" (past,present,future), when you are speaking of a past action that happened during another event is in process, the rules are simple:

1. When my mum came home, I was doing my homework (continues).
2. I was doing my homework (continues) at the moment when my mum arrived.

I was laughing when i was watching (is not correct) except you want to imply that you have been laughing as long as you watched the video.