Why do I need separate logins or accounts for EM-COP Incident and Training?

EM-COP Incident and Training are two separate environments or systems.

Incident is on a fully scaled environment to ensure things work smoothly for large numbers of users and incidents.  Training is on a lesser environment as it is not intended to support large numbers of users, exercises or training at the one time.

Training does have the same data layers and data feeds so it replicates Incident in some ways.

So as they are separate environments, you need to set up an account for each one.