@MVP maybe they made it mandatory now.
I opened my sons account 1-2 months ago and they didn't ask me to open any account.
Anyways no harm in opening your account too, I liked their interface and they do allow selling in pre-open which is one of the important criterion for me to choose the broker.
If you don't wish to keep 3 accounts, zerodha can be closed down as Upstox provides all the features which zerodha does plus fund withdrawal is possible in 1 day from listing while it takes 2 days for zerodha.