First post but I have been following this site and other security forum for quite sometime. Normally, you supposed to only have one signature based antivirus and a malware supplemental, but I found out that Quihoo TSE and Avast play nice together without any issue except for the additional memory increase with having two antivirus brand. I have play around with layered protection for years and always changing my security setup depending on changing risks. This is my current setup and free.
Firewall: ZoneAlarm firewall free with default setting
Primary AV: Avast Free with PUP/Harden Mode Aggressive
Secondary AV: Quihoo TSE with all AVs enabled / Registry and scanned file on open disable
Ondemand: all popular antimalwares apps
Anyway, I found both Avast and Quihoo barely use any CPU cycle once they settle in (done with the initial scanning of critical / startup system files). Only when I am installing or updating a program that when I start to see some activities from them; otherwise, only Avast behavioral blocking service seem to be the only process seem running in task manager. Also Quihoo take up about 30-40MB of memory when it does quick scan of a file during idle.
I would have to say this setup didn't have much impact on performance beside a small increase in memory usage. I run this setup on all my laptops and desktops at home, about 7 machines all.