Why does FZT653 a SMT BJT device have 2 collectors?
Both collectors are connected together inside the package. The one on the "top" (or tab) is normally soldered down to a an area of copper on the PCB to act as a heat sink. To achieve the maximum rating for this transistor (3W power dissipation) this copper pad needs to be about 50mm x 50mm (see the datasheet).

In practice you should connect both collectors together in your circuit.