imagine still further, that in ChildClass1 some object methods are defined, which i like to use in ChildClass2

As other people have point out you can do this by giving the full name of the method involved. However I'm really sceptical that this is the right solution to the problem :-) Having to do this sort of thing is usually a sign that there is something up with the way responsibilities are distributed between the classes.

If both classes need the same method then the most obvious thing, to me anyway, would be to put it in a common parent class.

If you give a bit more concrete info on what the classes do and why you need to shift methods around like this we might be able to give a better solution.