There are occasions when MSE will conflict with avast, there are already some cases of this in the forums already. There are some using it in conjunction with the avast beta trial as the beta wasn't fully functional.
However, the premise about two resident AVs still holds, don't, it will at some point bite you in the ass. Having two resident scanners installed is not recommended as rather than provide twice the protection it can cause conflicts that could leave you more vulnerable.