If I can read into your situation a little:

You want to setup a RADIUS server that authenticates against more than one Oracle server. When one site goes down, you want the users to automatically authenticate over to the second geo location. I architected this exact same setup for an ISP in the Southeastern US.

First, your RAS units probably have the ability to authenticate against a primary and secondary RADIUS Server. If not, you may want to look at a product called Steel Belted Radius. In this particular ISP's case, I specified two distinct geographic locations each with a 2 node OPS cluster. Each OPS cluster then used replication to sync it's changes with the other site on a 5 minute basis. Each RAS unit would then have the "closest" RADIUS server as the primary and the "farther" RADIUS server as the secondary.