How can I use modules from one ember addon within another ember addon?

I have two addons, one of which contains a module that I'd like to use in the other. How can I import from the first addon into the second?

I have tried a few different approaches, including exposing the required module in both addon-1/addon/... tree and addon-1/app/... tree but to no avail.

This is how it currently stands:

Addon 1

addon-1/
  addon/
    lib/
      my-module.js
  app/
    lib/
      my-module.js

The files:

# addon-1/addon/lib/my-module.js
export MyModule; 

# addon-1/app/lib/my-module.js
import MyModule from 'addon-1/lib/my-module';
export MyModule;

Addon 2

addon-2/
  addon/
    lib/
      some-other-module.js

The files:

# addon-2/addon/lib/some-other-module.js
import MyModule from 'lib/my-module';
export MyModule.extend(/* extending the imported module */);

# alternative version, also fails
import MyModule from 'addon-1/lib/my-module';
export MyModule.extend(/* extending the imported module */);

This results in an error when trying to build addon-2 for testing:

ENOENT, no such file or directory '/path/to/addon/tmp/tree_merger-http://...tmp/lib/my-module.js'

I assumed the trees would be merged and my-module would be accessible via addon-2's tree as when modules are used within apps.

Am I missing something obvious here?