the 'weakness too' effect wil not amplify the magic in the same spel. so you need to hit your target a 2nd time to get advantage of the weakness too magick spell.
Yes, that's why I said shoot the target several times.

However, to be completely clear, the WtP will effect the target even with the first shot. So will the WtM, but the WtM with the first shot will not do anything to the WtP on that shot. However, if you hit the target with a spell or hit it with a poison after shooting it one time, either effect will be triggered (within the time duration, of course). If you continue shooting it with WtP + WtM, you will easily multiply the impact of your eventual final shot. One hit kills even against tough enemies.