# How to use HKDF to combine two keys

I have heard that to combine two keys HKDF is better than using XOR function. But if you look at the HKDF RFC I could not figure out how to use it for concatenating two keys. What I can see is HKDF can be used to derive a key from another key. So I want to know how to use HKDF to concatenate two keys.

$$combined\_key = \text{SHA-2}(k_1 || k_2)$$