Multiple Clients Vs Single Client

Hi All,

One of our customer wants to add another company in the existing landscape.
There are two possible approaches

1. Add the new company in the same client
2. Add the new company in a separate client

 
The advantage is obviously go to with the option 1 as it involves less cost in the infrastructure/multiple implementations across clients.

But there are some problems related to the security aspects etc in this option.

Still the general practise is to add them in separate clients.

Now the question is what are the possible functional/technical issues in going ahead with the option one.

Regards,