Hi Tsvika,
I don't believe this is going to be possible due to the fact that artifacts are ranked against each other irrespective of what their parent item is. The actual ranking attribute happens on a very primitive form of the artifact that both features and user stories are descendants of. This would have to be something that happened within the UI to reorder children against children of another feature.
There is a fairly involved discussion from a couple of years ago here: Best Practices for converting portfolio item ranking to user story ranking . There might be something useful for a workaround in there.
This might be something that you could do via the App SDK; you can find more about that here: https://docs.ca.com/en-us/ca-agile-central/saas/for-developers
Beyond that this would need to be suggested as an idea on our ideas board. Please see the following article on doing that:
How to submit an Enhancement Request using the Ide - CA Knowledge
Anyone else have any other ideas?