Zerodha charges Rs 0 for equity delivery brokerage. You don't have to pay anything.
Intraday brokerage charges for each'successfully executed' order.
If both transactions are part 1 of the order, then you will only pay Rs 20. It doesn't really matter how many transactions you order in exchange.
If they are part a different order, separate brokerage charges of Rs 20 per order will be charged.