Is Multisite the right solution for me?

It sounds like you need two separate installs rather than multisite. Separate installs can still use the same database – although you have to use a different db table prefix on the second install.

Correct me if I’m wrong, but if you’re using different tables wouldn’t you no longer have matching content on the two domains? The idea is that my client can update both domains from one location and that the two domains will always have matching content (although different themes).

In that case, what you’re asking for won’t work. You cannot have WordPress generating the content for two completely different urls from a single install. It’s not even going to be possible using multisite as the sites would still have separate logins & separate content.

I’m surprised that your client hasn’t considered the duplicate content penalty that he will almost certainly incur from Google if he tries to go through with this idea.

Yes, I was planning on recommending a different solution anyway because of the SEO hit and other reasons, but I wanted to know if and how it was possible in case he wanted to go through with it regardless.

Thanks for the help.

The idea is that my client can update both domains from one location and that the two domains will always have matching content (although different themes).

Well.. you CAN do that with Multisite. You just have to get a broadcasting plugin to push posts from A to B every time you post.

Will this cause problems? Sure. It’ll make your site slower, it doesn’t stop someone from going to Site B and making changes, and SEO. It’s a very bad idea, though, and causes branding issues. Site duplication is confusing to your users.