If you want to be able to stop safely, in a straight line then get 4 winter tyres.
If you want to be able to corner safely, while breaking, then get 4 winter tyres.
Either fit 4 or fit none and drive your car according to the conditions. Fitting 2 is potentially dangerous as the car can too easily reach speeds that require 4 winter tyres for safe cornering and breaking. 2 winter tyres are not that much better than 'no' winter tyres at relatively low speeds.
A car with 2 winter tyres nearly hit me last year. The rear of his car swung onto my side of the road when he breaked going round a corner.
Have a look on youtube. Try searching for 'tyres on ice' IIRC.