Hi,
Yes, you can add multiple Analytics login companies to a single Marketing Cloud organization. You'll need to have Customer Care put in the request to our Corporate Provisioning Team. There can be some complications if you've already deployed the MCID service on both sites. Also, if you're using other Adobe products on each of the sites, they don't follow all of the same rules. For example, only one Target solution can exist within the same Marketing Cloud organization.
If you'd like to give me some more information about your current scenario, I'd be happy to provide more information around what you need to watch out for.
Cheers,
Jantzen