You and the flat buyers have to mutually agree on the resale price of the flat.
You may grant an OTP to the flat buyers at the agreed resale price.
If the flat buyers wish to purchase the flat, they must exercise the OTP during the Option Period. You have to wait for the OTP to expire before granting another OTP.
After the flat buyers have exercised the OTP, you and the flat buyers must decide when to submit the respective portions of the resale application to HDB.