It depends whether you are using older ga.js code or the new universal analytics (analytics.js).In the older code you can send data to 2 or more profiles simultaneously using this method:
var _gaq = _gaq || ;
You can use the 12345-X UA codes for any individual site and have their own isolated data and use the 34567-1 code as a master code shared by all of your sites. Please note that if you wish to track traffic between the sites using the same UTM cookies you have to implement cross domain tracking.
If you are using universal analytics then you can use multiple trackers out of the box: