6V is only just enough to power an Arduino from the Vin pin, and with a load of 1.2A, your battery voltage will drop to well under 6V. Either use a separate battery to power the Arduino, or use 5 or even 6 AA cells instead.
Although connecting BJTs in parallel for higher current isn't recommended, for switching applications you can sometimes get away with it if the transistors are of the same type, preferably from the same batch, and you use a separate base resistor for each transistor. But don't expect the current sharing to be even. I would never do this in a commercial design. For switching 1.2A it would be better to use a mosfet.