It is known that the problem of finding two disjoint paths such that the length of the longer path is minimized is strongly NP-hard, cf
Chung-Lun Li, S Thomas McCormick, and David Simchi-Levi. The complexity of finding two disjoint paths with min-max objective function. Discrete
Applied Mathematics, 26(1):105–115, 1990.
What about the spanning tree problem and the assignment problem?