Hello,
I have two tables the Main table and the Child table.
The Main table has primary keys and thye child table has the foreign keys refering to the primary keys of the Main table.
Can I make the foreign keys as the primary keys also in the Child table?
Or only the foreign keys ias enough.

if you make foreign key in your child table as primary key then you will loose integrity of your data because you can insert different data in your primary table as well as child table.
But if you define foreign key in your child table that mean you are ensuring that no other data except the primary key (in your main table) has can be inserted into foreign key (in your child table).

Now decision is your, what way you want to implement.

Yes, you can do that, but the question is why do you have two different tables with the same PK? If you are doing that for some extreme performance requirement then it might be OK, but it is not 3rd normal form.