ORA-01105: mount is incompatible with mounts by other instances ORA-19808: recovery destination parameter mismatch
When this happened to me I ran this command on both instances:
show parameter recovery
it showed there was a mismatch for the parameter db_recovery_file_dest_size. Probably I forgot to add "sid=*" to the alter system command one time when I increased the limit. Since one instance was up I could update the parameter for both instances with:
alter system set db_recovery_file_dest_size=700G sid='*';
The instance that was started must be shut down and started again.
This means that this parameter has to be the same across the instances during startup, but Oracle allows you to set them different with alter system.
As shown here you can correct parameters in spfile for an instance that won't start from a running instance. This is a favourite feature, and whenever I have to change memory parameters that requires restart of the instance, I keep one instance running so I can correct the settings from there if needed.