Does mixing two derived keys make a stronger key? And if so, how would I mix?
This answer is part answer, part comment requesting refinement of the language in the question. 

There is an assumption in the question that is not necessarily valid. 


  Using just a single KDF seems out of question to me if you really need secure and hard to attack keys


There is a counter example to the intuition that using more functions will make it more difficult: Suppose you are allotted 1 second of time per evaluation of the key derivation function. If you use scrypt for .5 seconds and then bcrypt for the next .5 seconds (or vice versa), your key derivation function uses less memory then just using scrypt for the entire 1 second. So if memory hardness was the goal, then mixing two functions does not necessarily improve security. Which leads to the next point:


  So if I use two derivation functions, one that is known to be secure but a bit weak and one that is very strong but not (yet) known to be secure, wouldn't this lead to better keys than just relying on one method?


First, there is a contradiction: What exactly is a method that is known to be secure, but is weak? How can we know it's strong if we do not know it is secure? If "strength" is somehow separate from "security", your definitions for the two terms would be appreciated.

Secondly, you have to define "better" keys: better in regards to what?  


  How would I mix the two keys? What would be a good, secure method for that


You have to define "secure": Combining hash functions is not trivial.

If you can assume that at least one of the outputs is psuedorandom, then there is little reason not to use the simplest method possible, namely, XOR. The output of $x \oplus y$ is at least as random as the more random of the two (otherwise a OTP would not work).

Unless for some reason, you want order to matter, then you could use a non-commutative operation such as concatenation and hashing. Unless you have an explicit reason for needing the properties of some other mixer, it is probably best to use the simplest tool for the job.