I discovered something yesterday that I find interesting. I downloaded the latest version of Malwarebytes and decided to do a full scan with it. While the scan was proceeding, an alert from CAV came up for the current file being scanned by MAM. It turned out to be a FP that has been corrected but it showed me that while you scan with MAM, the real time scanner of CAV is also coming into play. There is no need to scan with both if you scan with MAM first.
Happened to me too! 
Scanned a system (not mine) with MBAM 1.39, CIS detected a file (FP), I reported the false detection, and two hours later it was fixed!
That’s what sometimes happens with two scanners active.
This is why it is recommended to only have a single scanner active at a time… The two are basically fighting over the same files.
At best, it degrades scan times. At worst, it crashes your computer.
Edit: It’s really easy to disable CIS’s real time scanning engine while running another scanner from its tray icon.
CAV doesn’t seem to slow down MAM at all and I actually like the way it worked so I’ll leave it as is.