With AVG currently installed this option isn't avaiable (without some hassle), so installing avast also could cause conflict.
First (some years ago), I've used to use AVG mail scanner togheter with avast. Just before AVG 7 (free) release, I had no troubles. The other residents were disabled. After some problems, I just only use AVG
without even installing the residents.
I did not configure it later, disabling it (I think AVG could be disable with easy, not like you're saying David. Seems easier than avast to be disabled). So, I can't find any issue with avast resident and AVG not-resident (not-installed).
It's not for common users. No advantages on resident protection. So, I don't
recommend it. But, there is not a conflict after all.
In fact, I'll go further, in some systems AVG resident + avast! resident works. It's a rare combination of two antivirus runnning at the same time in the same computer. Again, it's dangerous if something goes wrong
Hope this help in anyway.