The usual way is to have a separate panel at each
of the two levels and to have a post between them.

Hmm, I can't add a new post at this point anymore. Concrete wall is solid/filled.


Is there a way to modify the fence panel?

Not that looks very good.


Is there any way to add a new horizontal support? Something like this:
https://photos.app.goo.gl/N7tcDehgkqsXGCi47
such that the middle support would hold the bottom (because the bottom would not be attached on the left at the step-up).