You should be able to use your standard picking method to complete T.O. 21935. IF the T.O.21935 was created for the full amount of the delivery requirement then the user would not be able to create the new T.O. 24123 with reference to the delivery. (SAP will not allow creating a T.O. with reference to an outbound delivery when there is an existing T.O. for the full pick quantities). Therefore after completing confirmation for the original T.O. whether you use you current picking method or LT12 the new transfer order should be canceled with LT15.
When a T.O. is created without reference to the delivery it will not update the delivery statuses so you should make that clear to the user.
If there was in fact still an open quantity in the delivery for transfer order creation (first T.O. was not for full ordered quantities) and the T.O. was created with reference to the outbound delivery with LT03 for example then both transfer orders should be confirmed....but I suspect user created second T.O. with LT01 based on your comments