I don't think you should run more than one scan at a time (other than having a resident scan running in the background and doing an on-demand scan).
You do not have to disable any parts of Avast to run your other scans. I actually have another resident scan [see Signature], but again, I do not run this at the same time I run any of Avast's scans. Keep in mind that Avast runs in the background all the time (resident protection), and this is fine.
Let us know if you have any additional questions. Thank you.