>But my problem is I have to restore two different backups to a single database. Is that possible at all?

You will loose the data from the first restore when you restore second backup. You have to find an intermediate way.

You can restore both backups as individual databases and then use tools like Redgate schema and data compare to generate scripts to merge the data and then take a full backup of the **merged database** and then replicate. This will be time consuming + your business will have to do testing on the new data to **stamp** it accurate. 

The database size will be huge due to the data volume.

You can follow : [**SQL Server Many-to-One replication**][1] approach that I pointed out and **Kendal Van Dyke's** approach from [**Reduce Transactional Replication Synchronization Time By Applying Snapshots Manually**][2] 


  [1]: http://dba.stackexchange.com/a/43995/8783
  [2]: http://www.kendalvandyke.com/2008/10/reduce-transactional-replication.html