That depends on which two. Since they would be performing similar
activities, expect that either (1) they will conflict or (2) one will not
really do much, because the other one always gets there first.
However, it is an acceptable (and even recommended) choice to have a
second AV software that is run only 'on-demand'. That's an especially good
idea for checking out downloads before installation and e-mail attachments
before opening.
I use Norton as my main , with AVG as a run on demand AV. The reason
for this is that Norton seems to have the better email scanning, but
has missed the odd infection that AVG has picked up.
