That’s right folks: Two condoms don’t offer double the protection. In fact, one or both condoms could tear due to the friction between the extra layers, leaving you without proper protection. So, if you’re thinking of doubling up to be extra safe, we recommend taking oral contraceptives with your favourite barrier method instead. That way you can be protected from both pregnancy and STIs/STDs.

Reference: http://goaskalice.columbia.edu/answered-questions/are-two-condoms-better-one/