I practice in CA; here you can start expungement proceedings at the conclusion of probation so you may qualify, depending of course on PA law. You should be aware that in CA at least DUI's stay on your record in the event you are again arrested for the same thing. In other words getting one expunged does not keep it from being a prior offense if you get busted for the same thing again. Since you are considering an officer's program in the military it is worth spending the money to have a lawyer help you sort this out.
In Pennsylvania you can only get your criminal record expunged if you went through the ARD program, have been deceased for 3 years, or are 72 years old or older & have not been in trouble for 3 years. Since you went through ARD (at least once) that portion of your record can be expunged. After you finish the ARD program, the statutes require your record to be automatically expunged, but not all counties actually comply with this. Sometimes you need to file a Motion for Expungement. If your 2nd DUI did not go through the ARD program, then you cannot get it expunged. Your only option would be to apply to the Governor's Office for a pardon. Good luck!