from Ms. Sun's blog
"I get this question all the time and the answer is no, you should not submit multiple SIRs. The UCOP processes a report after the SIR deadline that checks duplicate SIRs and then the individual UC campuses decide on whether to take action (most of them do). Most likely you will receive a sternly worded email asking you to decide where to enroll by a deadline. If you don't respond by the deadline, your acceptance will be withdrawn from both campuses where you SIR'd. If you are trying to buy time because you are not sure whether you can fulfill the Provisional Admission Contract/Conditions of Admission, understand that you are buying a week of time at the most (maybe two if the UCs are busy). The exception is when you are waitlisted at or appealing to one or more UC campuses, in which case you may SIR with a backup UC campus. Just make sure you withdraw your SIR from the backup campus promptly when you are admitted from the waitlist or appeal."
Basically they won't cancel your SIRs if you respond to them in a timely manner after they contact you asking you to make your decision. If you respond after the deadline though, they will.