Don't mix the two different applications types, AV are different from AS (anti-spyware), whilst there are some areas of crossover they are different classifications.
So you should only have one resident, on-access scanner in each type, never two resident AVs, or two resident ASs.
You can add on-demand scanners to each type to provide a back-up/compliment the resident application.
Some people have managed to get away with two resident AVs but this is more by luck than design, AVs install low level device drivers to intercept/hook files to be scanned before they run and it is these low level drivers that can conflict. This could be as simple as only duplicate scanning of the same file to a full blown conflict freezing/locking the system as they fight for control. This can actually happen very early in boot too resulting in a failure to boot.
So there will always be a risk of this conflict even if until now they have got away with it. Even if they disable one resident, the low level driver still runs unless you do some surgery to the registry, etc. to disable that too, to have to go to those lengths makes it just easier to uninstall one of them.