[quote name='Sulmona']very ymmv, not all csrs can override themselves[/QUOTE]
Most of them call a manager, and they say "Hey, it's not accepting the second coupon, can I just over ride it" and the manager, who usually has better things to do, just says "Sure, go for it".
They bring out the yellow piece of paper, write all the info down, and that's it.
Only once out of 10 + tries did I ever get denied because it was extremely slow in there that day, and the girl read the coupons and noticed where it said "Cannot be combined with other coupons".
Your best bet is to hit Best Buy when they are busy and pick the crowded register, since they just want to get you the hell out of the store at that point.