Chlorpromazine is a pretty powerful first-generation anti-psychotic, and it will completely swallow the effect of the diazepam. Not that you won’t feel the combined effect of both, but the chlorpromazine will overwhelm it. I would expect seroquel to act similarly.
Maybe the diazepam will somewhat reduce the akathisia from the anti-psychotic and make it slightly less anhedonic. But I wouldn’t risk a benzo addiction for that purpose.
To be fair, both Thorazine and Seroquel should be sedating/anxiolytics enough on their own. Depending on indication, I would suggest only one or the other, as both at the same time is probably unnecessary.