Can I use two ED3538R without an ED3538T for communication?

Answer:

Yes. In PoL (Power over Link) application, the topology looks like this:
[ED3538T: local mode]---------------PoL-----------------[ED3538R: remote mode]

If you don't want to use PoL, then you can use above topology or you can use two ED3538R for communication without using an ED3538T.
[ED3538R: local mode]-------------------------------------[ED3538R: remote mode]