Is it possible to attack ac2 without destroying the ac1?

You can attack any Alliance center as long as you can reach it from an alliance owned plot. This can be done by building a bridge to it of plots from any that you already have in the state.

If you do not have plots though then you can only start from a Rally point. When war starts, though, you can only place a rally point in the outer area of the state which will be where the enemy AC1 is. Once AC1 is destroyed you can place a rally point in the second area close to AC2. It is always worth checking to see if you can bridge to the enemy AC2 (or 3 or 4) from your own plots in case it is quicker that heading to AC1.