Making a backup of my wallet
If you made the second transaction to same Bitcoin address so yes you will not miss that second transaction even if you made the transaction after the backup. 

if you made the transaction after the backup and you made the transaction to a different address you will also get any future transactions for two reasons: 

1- Bitcoin Core using HD wallet so you have one master private key and they derive all your future addresses from that one seed.

2- if you are using old bitcoin wallet or HD wallet set to 'OFF' you will still have future transactions but not forever and that because of the limit of keypool future. 


  Keypool New public and private keys are pre-generated and stored in a queue
  before use.
  
  This pooling feature was added so backups of the wallet would have a
  certain number of keys that would be used in the future. By default,
  the number of entries in the queue is 100[1].