Concatenating hashes
It is unclear what you are aiming for? 
If you are trying to get a hash function collision resistent even in the face of a malicious adversary? murmur is no good regardless of concatenations. 
You need to use a cryptographiclly secure hash such as something from the SHA-2 or SHA-3 families.

If you are talking about a non adverserial setup, murmur may suffice. But since modern cryptographic hashes have so fast implementations I usually recommend them anyway and have better piece of mind.

Regardless, both murur and the SHA families are defined on arbitraty length input so you can apply directly on concatenated input.

If you already have hashes, their concatenation(at double the output size) is at least as collision resistant as the underlying hash. The second construction at half the size also seems secure when a secure hash is used.