What happens when relative and absolute lock times are used in the same transaction?
When a specific locktime is specified in the nLockTime field in a transaction. It means that said transaction cannot be broadcasted/be accepted by miners until either the blockheight or unixtime has elapsed. 

On the other hand, when a OP_CSV is used, it means that a transaction referencing said input cannot be spent until some blockheight or unixtime has elapsed. 

If we merge both and use them together, basically, we will create a transaction that cannot be broadcasted to the Bitcoin network until some nLockTime has elapsed and it also cannot be spent until some blockheight or duration of time has elasped. OP_CSV specifically targets the spendability of a transaction, whereas a time-locked transaction simply prevents some transaction being broadcasted to the Bitcoin network until its locktime has elapsed.