why is "ti" required in this sentence?
Not required. "Ott játszunk, ahol játszotok." should be correct.
I'd say "ti" is more required than "játszotok" here. The same action being involved is almost implied, no need to put emphasis on that - or, better said, we have all reason to remove that emphasis and focus on the "who" part.
Ott... ahol= where
Right. It depends on the sentence.
Is it wrong to say "We play there, where you play?"
In English? I think so.
I mean, is that a bad way to translate it?
I wrote: "We play there where you are playing"
I think "We are playing where you are playing" would be the best translation
Not in English - it is not common, but no problem
What's with the "ott"? Transl. "We play where you play." There's no, "there" there.
In Hungarian you need the "ott" for the "ahol". The pairing is required in Hungarian grammar.
Why not "... ahol jatszol"? And, doesn't the "...tok" ending in Duo example signify "I", not "you"?
-tok is second person plural. First person singular doesn't have the "t" ie just -ok.
Is "itt" ever used in place of "ott?"
In sentences like this, not really.
"We are playing there where you are" not accepted
Why is there an "o" before the "tok" ending?
Because it is too hard to pronounce three consonants - tszt
So is there a rule involved there?
Yes - if there are two different consonants add a linking vowel. (And in some cases with a double consonants) Also if the verb ends with -ít.
Triple consonants are very rare in Hungarian, you always have to add lying vowels when you can (same for nouns, e.g. with the thing=a cuccal/cuccaval while cucccal would be the normal way to say it)