As i said

TableA C1 uniqueidentifier (PK), C2 uniqueidentifier, C3 ......

PK is a cluster index

TableB C1 uniqueidentifier(PK) , C2 uniqueidentifier (FK_TableA_uniqueidentifier ), C3 ......

Column c1 PK is a cluster index and C2 is non clusterindex

Trying to insert some million of rows

C++ is the used to insert the data.

In C++ the "sql statements" are used to insert the data.

Since there where no stored procedure used , unique identifer is kept as source for uniquness in the table.