The effects may somewhat mask one another. So taking them together may make you feel less drunk, and less amped up, but may provide for a less anxious amphetamine high. Also, they both cause you to become dehydrated, so you may become even more dehydrated, causing a worse hangover.
I think that amphetamines absorb better if you take a base like sodium bicarbonate with it, so since beer is slightly acidic it may slightly hinder the absorption of the adderall. A lot of people report stomach contents relative to pH as a large factor in feeling the effects of amphetamines.