Thanks for your answer.
However, the two networks are of course disjoint, because that is the clue of that set-up. From a routing point of view, the networks are disjoint as soon as the both netmasks produce distinct network numbers. Following classical sub-netting and choosing i.e. 192.168.42.0/25 and 192.168.42.128/25 instead of the IP ranges in my actual set-up would not make a difference, because the two networks then were (and shall be) still disjoint.