Potential weaknesses when combining hash functions?
For generic weakness no.
Consider two weak hash functions H(x) = 1
This isn't preimage resistant but obviously concatenating a 1 to a hash function does not make it easier to find a preimage to it.

And consider H(x)=x
A very different kind of weakness and concatenate the message or parts of it would be super helpful in finding a preimage for the other function.