Well I don't know much but I believe that when Ad-aware is scanning, Ad-aware is opening up those files that it's scanning to detect any spywares or malwares. Since avast! Standard Shield detects and scan all files that are open, running, etc, avast! is scanning along with Ad-aware. I might be mistaken but that's just my hypothesis.
however during a scan with adaware avast picked up two trojans very odd.Considering i was not scanning with avast.
Avast! Standard Shield again scans all files that are open, running, etc. I'm assuming that when Ad-aware was scanning a few files, avast! detected some of those files having Trojan viruses.