I think a better way to accomplish this would be to use one account, and define a view in Analytics that includes external IPs. That's the way we're handling this need.
In a nutshell:
In Analytics, Administration, select the account and property you need to filter. Create a new view (e.g. External Only). Select Filters, and add one that excludes IP addresses starting with the value corresponding to your private network range (e.g. 10 or 192.168).
Once you have the view saved and selected, go back to reporting and you should only be seeing external hits. If you need to be able to view internal only, you can easily set up a view for that too.