You can choose anyone from both collectors to use in the argument but make sure port 9998 or your own custom port should be open for both the collectors to get the benefit of FO/LB.
You can watch the below video that how agents work and how to install locally or remotely.
Let us know if you need further assistance.
Thanks Babar Q for reply , It will help if could explain us how this works as we provide one collector name while doing argument , how ? if that collector go's offline how dt agent send's data to DT server from the failover collector server.
First of all port must be opened for both collectors from all the agents/hosts to the collectors.
Secondly, once you will have a collectors group then each connected agent will have the collectors list with them, therefore, in case of failure of any collector the connected agents to that collector will silently move to another collector which will be available in the collectors list and the best thing is that it all happens automatically.