extra protection

If our contraception is rated at 98% and we use it properly,could we make it more efficient by say using foam spermicide rated 85% on its own, that if users have sex 150 times per woman year  is only one failure per 1000 sex acts. Our question is therefor would we be 1000 times less likely to fail if we added using foam with our chosen contraceptive? Two methods instead of one!!!!