I quite agree with AdityaSomani.
Usually when you check online, you can add anyone on the same flight to your check-in itenary, you only need their surname and Amadeus number. Just be carefull that sometimes available seats depend on the ticket vendor; once we weren't able to fly together just because some bought the tickets through a different website than others.
The very same thing applies for checking at the airport: they try to check a group together if the people come to the drop-off counter together. Just it's advised to be there in greater advance so that coupled seats are surely available.
Therefore it should be sufficient to buy both tickets at the same place (either shop or e-shop), have both tickets in the same travel class, and do the check-in together. Please remember that the cheapest option is often not available for one-way tickets, therefore getting the same class might be difficult. Because of this, I would recommend consulting an office before purchase.