Do I simply drill two more hole or will doing this result in weakening and possible cracking of the plate.
Yes you just drill extra holes in the plate. The plate isn't subject to any significant movement so you can drill to your hearts content and turn the plate into swiss cheese if you want before any weakening would affect it.
I was lucky to receive the US plates with my car so used it as a template for the drilling so best you make up a cardboard template of your plate and mark up the holes on the car onto it, then you can attach the template to check your plate will fit nice and square, you don't want it looking crooked.
Mine came with plastic inserts for the screw holes that the screws would screw into, not sure how it is for your model, someone else here may know what's best.
s