My opinion: don't do it. The difference between your first and second choices aren't so great that it's worth ruining your reputation. Most ESPECIALLY if they're in the same city, since word gets around. Plus, if you do this it does reflect poorly on your school, so in a sense you're screwing over future generations.
Although you could try asking if they would like to split, I guess. That's not technically reneging? Still sounds shady though, who knows how they'd take and it if it reflects poorly on you at the firm, I would expect that would increase your chances of getting no-offered? Who knows though. I'm just talking out of my rear end.