Can I verify the same sending domain for 2 different customers account?



I have 2 customers, partner in their business, and they want to use their own customer account but with the same sending domain. To my end, I require the domain verification for each customer account
Then, in the first account I verified the sending domain with success but in the second accound I couldn't, sure, because the sending domain is already in use in the first customer account...
So please:
1. Is it possible to do that ? and how ?
2. If not, how can I turn off the sending domain verification for only one account?

Thanks a lot for your help :)
Best Regards,