Barbara

You wouldn't damage it per se - but they would go out of balance from what I understand - when adding the second battery I have read its a good thing to disconnect the original when at 100% and then charge the new one to 100% and then hook up both - it will balance itself over time though anyway probably
 

However, shown like this on their website 🙂 

 
 

Other people will also confirm - but yes as far I am aware your pic with the changes, is the correct way to wire it up.

The Victron should see two 5kWh batteries in parallel as a single 10kWh battery. The two should charge and discharge together.

It's correct. It's done to make sure that the current flows equally through the two batteries. Do not shorten which ever lead you move to the top battery in the stack.