Can i restore more than one msgstore.db.crypt7 file in whatsapp?
As you can read in the wahtsapp faq, 


  Current chat history will be lost upon restoration.


Anyway, you could follow these steps.
For every older conversation:

1)Use the "email conversation" function:


  To export a copy of the history of an individual chat or group, you
  can use the Email Chat History feature:
  
  Tap and hold on the conversation or group. Select Email conversation
  Choose whether to Attach Media or not. An email will be composed with
  your chat history attached as a .txt document.
  
  If you choose to attach media, the most recent media sent will be
  added as attachments.


2) Send the composed email to yourself.

After you did that for every older chat, restore the newer ones using your backup.
This way:


You will be able to read your newer messages on whatsapp normally.
You will be able to read your older messages opening the attached .txt file in the emails that were composed