It is always a rather nightmarish thing to takcle the use of the imperfect, for native speakers of languages where there is no imperfect.
In your example, two activities are happening simultaneoulsly in the past, none of them making irruption into the other: you have two solutions:
Pendant que j’étudiais, tu as regardé la télé
Pendant que j’étudiais, tu regardais la télé.
What helps here is is you say: “you WERE watching TV” (not “while I was studying, you WATCHED TV”)
The continuous “were watching” would make me translate both in the imperfect.
But, if in your sentence, you would equally say “you watched TV” Things get a little more complicated, so I add this longer explanation, in case it is of use to you.
If you wrote “while I was studying, you watched TV”, then in French both imperfect and passé composé would be an acceptable choice grammatically, BUT using two imperfect tenses would have a different use/feel, for a French person, depending whether you are talking about a REPEATED OCCURENCE in the past (imperfect then takes over, because of the repetition), or a ONE-OFF occurence in the past:
To test that, check whether your sentence could work as: “while I would be studying, you’d watch TV”.
If that doesn’t feel right because it only happened once, then move on to the next test: asking: “and then what happened?”
“It was on Monday. while I was studying, you were watching TV”. You feel the question coming: “And then what? It started to rain? the doorbell rang? I asked you to put the sound down?”
If you use both verbs in the imperfect in French, for one-off situation, than you are making both activities the “context in time” for something to happen (and that event happening will definitely be in the passé composé).
“I was studying last night, you were watching TV... A French will ask: AND THEN...what happened?
“The cat threw up”
“The phone rang”
“We got hungry and both decided to go for a pizza”
All of those would be in the passé composé
If nothing happened and you are just saying what you did the night before, then a French person will translate your sentence and use a passé composé for one of the two activities:
Pendant que j’étudiais hier soir, tu as regardé la télé: the context was my studying.
Pendant que tu regardais la télé hier soir, j’ai étudié. The context was your watching the TV.
One event, one context, that’s where it gets interesting: both actions lasted the same and happened at the same time, but you are saying different things: either that the past thing to report was your studying and it happened while the other was watching TV, or the past thing to report was that the other watched the TV, and that happened in the context of you studying.
I hope that helps and doesn’t make it even harder!