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As far as I am aware this is not possible, a domain name has to resolve to one IP address, the subdomain is handled by the server.
As an alternative you can set up a virtual host (subdomain) on one of your servers and rewrite all requests to the second:
<VirtualHost *>
ServerName sub.domain.com
RewriteRule ^(.*) http://www.domain2.com$1 [L]
</VirtualHost>
I think that right anyway!
Andrew
To illustrate the point think of sites that have www1.example.com, www2.example.com... These tend to be different machines running an identical webserving environment. Taking it one step further, mailservers are often separate machines from webservers.
There's no room for DNS 101 in a post here, and I wouldn't be the best to write that, a little Google research should bring up plenty of resources.