You understand right!

Biggest problem that can happen is that you open a file and one of the AVs starts to scan it because it has been opened. The act of doing this triggers the other AV to scan because the file has again been opened. This has the effect of opening the file so first AV scans again. Ad inifinitum - infinite loop that just burns resources.

Obiously if they are clever and well behaved then they will avoid this - but it doesn't take much of a coding error to cause major issues.