What happens to the system is Plan B is taken more than twice a month on top of borth control?

Irregular bleeding. If you are taking a regular birth control pill daily, you should not need plan b. But if you have taken it twice and have not missed any pills, you may not see any change in your cycle, or you may see irregular bleeding. You should consider using condoms and spermicide in addition to your birth control pills.
Good advice so far. You don't need to use emergency contraceptives if you are consistently and correctly taking oral contraceptives. Plan B is to be used after episodes of failed birth control. Using it may cause side effects.