Quote from: khaled0111 on October 31, 2019, 04:25:22 PM

Logically the second transaction must pay more fees, otherwise miners will pick the first one from the mempool.

This can be a problem if the miners pick the first transaction (for example if the second transaction has the same fee (sat/byte) or for some other reason), then the second transaction is not valid anymore and it will fail and you have to redo the second transaction. If you want to make sure that this doesn't happen, then you have to set a higher fee for the second transaction which will negate the benefit that the "batch RBF transactions" has in the first place (save some fees). This may still work if you only slightly increase the fee for the second transaction but still you cannot be sure that this transaction will be confirmed first.