Without using a load balancer in front of the RADIUS servers, the only way is to put one server first for some network access devices (NADs) and another server first for others.
As long as the first one in the list is marked up, the NAD will use it.
A few devices (e.g., Meraki) do allow you to specify round-robin RADIUS selection. Reference.
A true load balancer (or "application delivery controller") allows much more intelligent server selection - using least connections, server weighting, etc. in the selection process.