Can i use the proof of no claim bonus on 2 cars?

Not unless both cars are insured under the same policy.

Other than that, if you use your bonus proof on one policy for one car you cannot use it on any additional policies for other cars.

Many insurers will still take it into account though if you ask and will offer an introductory discount; the best discount will likely be from your current insurer, but they might not offer the cheapest insurance deal so even with an enhanced introductory discount they aren't necessarily going to be cheapest.