Secondary MX Question

Hi

If I have one domain name and two sets of mailservers in the format of:

primary mx on my hosting account.
secondary mx on a backup account.

Do I need to have the domain dns hosted on both sets of dns, ie
hosting account
secondary account

Or will the backup mailserver work if the domain is only 'on' the primary dns account, ie:
hosting account

I think I've got it to work in the previous example but need someone to help me clarify the situation.

Thanks

 

 

 

 

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