Hi
When running on-demand scans with Windows Defender or MBAM to get a a 2nd opinion, is it necessary to turn off Avast?
No its not… You can leave it on… 
Turning off any real time shield should be done only with extreme circumstances. With avast turned off is another way to ask for trouble. I would leave it on except on advice of an expert. If an “evangelist” asks you to do so, it’s for a reason. I would leave avast “on” for the protection. “Off” means no protection. “On” means protection.
Windows Defender is a joke honestly. Your better off with these and all are free.
When I do Any scan I first update the On Demand Scanner which ever one you prefer…I prefer Malwarebytes or SuperAntiSpyware (SAS) and Then I turn off my Internet …And say I’m Using Malwarebytes…I would right click Avast on Taskbar and disable until computer is restarted…If you do want Avast to be ON, I guess that Avast scans “Malwarebytes Scans” or folders opened with Malwarebytes, and the Avast icon turns…If Malwarebytes was to catch any malicious files or trojans and they were in the Avast Database… Avast would pick them up Faster or first…But if you have Avast turned off while Malwarebytes is scanning…The Malwarebytes scan finishes much faster…Actually it boots at full speed…I’ve even scanned with Avast and Malwarebytes together at the same time …The only thing is it takes forever…Ive also done a Custom scan with Avast together with a full scan on Avast…Takes forever…But works…I do have ClamWin Portable on my USB…And since ClamWin is an antivirus Scanner only…I turn Avast off when It scans…
Cheers !!!and safe surfing!!!
No need for any of that Daris. You never have to disable your real time to use an on demand scanner. As far as disconnecting from the internet. Thats only necessary if you have a rookit calling home that you are trying to kill
Ya, You’re right Dieseleman…I just find the Scans are Faster, but either way…can’t go wrong…
Cheers!!! and safe Surf!!!
The reason why its slow when you run 2 scans is both these applications - Avast and MBAM access the hard drive at the same time which makes both slower… You need not turn off anything, it wont affect the speed as far as I know…
As dieselman said theres no reason to switch off a resident protection to run an on-demand scan… Theres nothing to worry about, won’t be any conflicts 
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.