order by when merging is pointles, you can always do order by when geting data from merged table
only to update the merged tables
The key here is ignore
As per key in your merged table, which should be the same as in source tables, by my recipie, duplicates(that is, records that you already imported)
will be ignored.
so you would do, to do update only
insert ignore into students_merged select * from db1.students_contacts;
insert ignore into students_merged select * from db2.students_contacts;
and run that any number of times and it will transfer only new records, that is, if you set key(index) right & up to your desires.
If you don't understand (which I think you dont) what i'm telling you, paste here create code of students_merged table.