I tested this out and you do have to create the global collection with a two step process. The first URL you are going to use to get the unique Global Collection ID. You can do this with the following URL:
http://localhost:8080/spectrum/restful/action/0x10474?mh=0x9600000
Your going to get a response that looks like the following:
<?xml version="1.0" encoding="UTF-8"?>
<action-response error="Success" xmlns="http://www.ca.com/spectrum/restful/schema/response">
<attribute id="0">56786f82-44da-1007-0351-00505686075a</attribute>
</action-response>
What you want to do is take the attribute value which contains the unique global collection id and we are going to use that to populate the 0x12e56 attribute vaule in the following URL:
http://localhost:8080/spectrum/restful/model?mtypeid=0x10474&attr=0x1006e&val=Branch-2&attr=0x12e56&val=56786f82-44da-1007-0351-00505686075a
When creating the second global collection you will need to send the action to the server again and use the new global collection unique id to create the second global collection