There are a number of different options depending at which stage you want to send to the 3rd party. Are the logs already configured to send to a Splunk forwarder of some kind? Is it collected via syslog-ng + written to a file, or just ingested via a tcp/udp input?
If not that, one option is having the RADIUS server point at a virtual IP, and have the 3rd party load balancer mirror the traffic.
If you're already collecting this as syslog via syslog-ng or something similar: In your outputs.conf, you could configure data cloning. So ingest the data, and send it to multiple destinations. If this is on a heavy forwarder, you might have to configure indexAndForward=false globally, which might affect your other data. If you're just using a universal forwarder, you should be fine as it can't index the data. See the following, and look for the cloning section: http://docs.splunk.com/Documentation/Splunk/6.3.1/Forwarding/Configureforwarderswithoutputs.confd