Primary and Secondary machines are the same. Once the synchronization is completed, it don't matter which one is used, the functionality is the same.
This is exactly correct. Functionally the two boxes should be the same, the only considerations to keep in mind are
- Depending on how you deploy this the secondary box might not be as beefy as the primary
- If you are setting up DR, meaning failover across the WAN to another site, you would not want to have Orion on Site A and additional Poller on Site B
- Depending on how you deploy the secondary and it's communication to back end storage, you may see some perf hit. Ex. Primary Orion server has direct GigE connection and secondary is on a VM on an ESX host which has many other VM's so it is fighting for resources and bandwidth