I don't know about how pills/IUD interact, but for the sake of discussion, let's assume they don't (do not assume this is the reality, though).

Multiple overlapping birth controls will have diminishing returns. A condom is, say, 98% effective. On average, 1 in 50 times you have sex with a condom on, the woman will become pregnant. Even if that statistic is true (that sounds like a high rate of pregnancy if you ask me), that means any other contraceptive like the pill/IUD/whatver only has 2% of the remaining chance to get pregnant to deal with. If another form of contraceptive is also 98% effective, and the way it stops pregnancies is in no way the same as a condom (a condom is a physical barrier, IUDs affect hormones/egg production/something else - I'm not sure how they work, they're just not at all a physical barrier), then that second form of BC is only effective against that remaining 2%. Which is not a whole lot to worry about.

Although, if abortion is totally out of the question, turning that 2% in to .02% is the difference between a kid and not in a huge number of cases.