Lets consider the following situation: Bootlayer (module Core) is a main application - child layer A (module A, module Foo) is a plugin A - child layer B (module B, module Foo) is a plugin B.
If I understand you correctly, you would add module Foo to the module path of both child layers? In this case, both layers would have access to a class called Foo, but it would not be the same class in both layers. This makes sense if you consider that both layers might just as well depend on two different versions of module Foo, so they cannot share the same loaded classes. As a result, your code would run into a ClassCastException.
and in plugin B I do var foo = (Foo) coreService.getInterPluginObject();
However, note that the JPMS is not limited to a single parent layer. You could add a separate layer containing Foo, which could be an additional parent for both, layer A and B. Depending on your context, plugin B may also declare a dependency on plugin A, and therefore get child layer A as a parent. I don't think that I can give you a proper recommendation as I don't know much about your project. (Also not that I am NOT affiliated with the OpenJDK project, I am just a fellow subscriber to this mailing list.)
