Classic case of joint honours departments not cooperating with each other...
I'd wager the practical is more worth it, as that's something once you've missed... you've missed it. If the creative writing is just a lecture type thing then you can get the notes afterwards.
Might be worth speaking to the creative writing lecturer
a) to tell them why you'll be walking in late all the time
b) any work to do before that lecture so you're not out of the loop during the second hour
This won't be the first time it's happened. Speak to the SU/council officers and push it as much as you can, although I doubt it will change anything this year.