Sure? Why not a pair of 1 1/2 ohm in parallel....or a pair of 1 ohm? Many practical combinations exist to put less than 1ohm in series w/the tweeter.
Just think about the resistor in this fashion:
If the driver is 4 ohms, and you put 1 ohm in series with it, roughly 20% of the power will go away...as heat, while the driver gets less power and doesn't play as loud.
You could put 0.1ohm in series and still get an effect, though the 2%+ of power now being dissipated by the resistor may not make an audible difference.
Just think about the resistor in this fashion:
If the driver is 4 ohms, and you put 1 ohm in series with it, roughly 20% of the power will go away...as heat, while the driver gets less power and doesn't play as loud.
You could put 0.1ohm in series and still get an effect, though the 2%+ of power now being dissipated by the resistor may not make an audible difference.