In my opinion there is no problem. Cuz autoattack have attack coldown not cast time.
If i understand you? To auto attack and do a casting spell between the coldown of your swing is possible on all classes. I belive. Harder & easier depending on class. Ofc
There is ways to increase your dps with timing a spellcast to your attack speed.
Then some what Undertanker mentions, To start cast in your autoattack-animation and you will hit twice after spell. Common thing but peapole understand it different.