I see. You mean it's the casting of the second spell that's not coming out properly. I'll do some in-depth testing when I'm back in the office on Monday, but to answer your other question, you can set any spell on the hotbar to use cursor targeting by right-clicking on the spell on the hotbar, going to Target, and picking Cursor. Also, if you have a spell set to Target:Current and you are not targeting anything (the golden ring spinning around the mob's feet), it will cursor target. If you are targeting something and you don't want to target it any longer, you can press Esc.
My typical set-up when I'm fighting is to set my Mouse Wheel Up to Target Previous Enemy and my Mouse Wheel Down to Target Next Enemy. That way I can roll the wheel any direction I want to get the target I want to cast on, then get to business. I also got one of my side mouse buttons (mouse 4 or mouse 5?) set to Target Nearest Mobile for those times when I just want to get the closest thing to me. That's just my personal playstyle, though. Everyone probably has something unique.