Question about second preimage resistance of hash function combiner
The answer you are looking for is that a pre-image/2nd pre-image attack on two combined hash functions is at least as difficult as an attack on the stronger of the two hash functions.

Take a look at Joux's paper on multi-collisions. There is a section on pre-image attacks for concatenated outputs. (I suggest reading the entire thing. It's one of those rare short, to the point, easy to read papers.)

The attack described in the paper has complexity of:

$n_g2^{{n_f}/2} + 2^{n_f} + 2^{n_g}$

For two hash functions of length $n_f$ bits and $n_g$ bits where $n_f \leq n_g$