How to share files between two different domains.

I have two companies owned by the same person. He wants to keep the companies and domains separate but he wants to share a certain folder/directory between the two companies and all files within it. What is the best way to do this?
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DP230Network AdministratorCommented:
Build a public server (with public IP address) to store the data, then map to that shared folder using public IP address and proper credentials.
Naveen SharmaCommented:
You may access resources on one domain while logged on another domain even if there are no trusts between domains. When accessing resources on foreign domain, user will be prompted for credentials. She must supply credentials in the form user\domain + password or upn (user@domain) + password.

Sometimes if the same username exists in both domains and passwords are different, the user might not be able to access resources. In that case map network drive.

Refer to below articles:

How do I share a folder across a domain
File sharing between untrusted domains

Apart from this, enable auditing for monitoring else, try some active directory auditing solutions to keep track whats going on your IT environment along with who did what and where with detailed report.
Dariusz TykaICT Infrastructure Specialist Senior Commented:
First there need to be connectivity between networks where both domains exists.
Second you need to setup a trust between those domains (proper DNS configuration is necessary for name resolution).
Third - when trust is in place you can assing permissions on local resources in both domain to users/groups in second domain.
Some articles that can help you:

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aandoAuthor Commented:
Both are correct. I think the Trust is harder on the front end but easier on the users in the long run.
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