I would say that technically it's not different than other federations.
But as it's 2 Essbase cubes, isn't it easier to just setup a new cube using partitions to merge the 2 parts of the cubes you need? It's maybe easier and also better from a performance point of view as I guess a federation between 2 cubes in OBIEE means that the BI server will do the work (and here you can have problems because your cubes contains the aggregated data, but how are you supposed to merge 2 aggregated data?).
Doing it in Essbase with partiitons will make it transparent for OBIEE and Essbase is good with that.
Additional to what Gianni said, it's not about "joining things". OBIEE isn't a database and federates queries across sources without join definitions but rather uses conforming dimensionalities. If you can push things back to the cube though it will be even more transparent than doing it in the RPD.