Suppose there is a contactless smart cards system using the MIFARE technology, can a smartphone with an NFC also be used as a smart card concurrently? By this I mean can a smartphone also be used interchangeably with the smart card. and if it does, how do the balance on the card (if it exists) be updated? after I used the smartphone to pay for something the balance on my account must be depleted right? and how does the card knows the most up-to-date balance?
You would need a smartphone not only supporting NFC, but also MIFARE, which is one of the most successful "technologies" built on top of NFC.
But if we generalize your question to a contactless smart card plus a NFC-capable smartphone, the answer would be: it depends totally on the infrastructure, on what parts of the application are implemented on the cards and on which are implemented on the backend. For example, in an account-based ticketing system you would be able to use your card or your phone indistinctly, but if the balance is stored on the card, you would have possibly two different balances on each device.