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How do I make a .htaccess script so it allows only mine and my other buddy's IPs to access the ACP?

All others should be denied 403 forbidden.

Thanks in advance!

This is what you need:
http://www.javascriptkit.com/howto/htaccess5.shtml

One more question, does the .htaccess file should be with .txt extension or what?

No. The .htaccess file must be just like this:

.htaccess

Nothing more, nothing less.