it would look something like this

alter database add logfile thread 2
group 3 ('d:\oracle\oradata\redoXXX.dbf') size 254m reuse, group 4 ('d:\oracle\oradata\redoXXX.dbf') size 254m reuse;

alter database enable public thread 2;

...notice that thread 2 is the key here enabling the logs for the 2nd instance