I'm not sure if it's my mouse or what, but half the time when I try to press both buttons to double cast my character starts doing 2 separate spells instead (one left, one right). I then have to abort the spell cast and try again. Is there a trick to this that I'm missing?

First check if you have the perk, if you do then try to set the right click to a different key and try that if it does not work i am clueless

I definitely have the perk, because it does work half the time. I'll try remapping, but I don't think it'll work because both of my buttons do respond (as evidenced by both of my hands starting casting), it's just that the timing to get it to double cast seems to be really tight. Perhaps there is some setting I can adjust.