So are you really sure that avast and MSE play nicely together then. Having two resident AVs is likely to lead to conflict, when will not be at a time of your choosing and you don't know the consequences.
So far I haven't detected anything other than a slight slow down. But as in my signiture, its just on my test machine I play with. I'm not going to try it on my everyday machine. Avast has been my go to AV for several years now and I don't expect to change.
However, I did add Sandboxie to my main machine after learning how to configure it and testing the capabilities on the test machine. My main computer has multiple users which brings the expected problems. If I was the only user, I doubt I would have bothered.