can we use two different hash functions in single hmac? Can we use two different hash functions in single hmac..for ex' for inner layer hash function md5 and for outer layer hash function sha1
This is exactly what was done in early versions of HMAC. Of course, there would be no reason to do this with an already broken hash function. Note that MD5 is completely broken and SHA1 is close to, so neither of these are great choices.