The other terminal most likely was a Cabcharge terminal. The driver needs this to process Cabcharge cards. Cabcharge is an unregulated card instrument which means non Cabcharge payment terminals , such as the CabFare terminal can not process this card. Therefore, the driver will have two terminals.
For peace of mind, when using a card other than a Cabcharge card, insist that the driver uses the CabFare terminal. This will guarantee that your transaction will be secure at all times.