was looking for the answer to the same problem.
Scenario:
3 different clients with Help Desk access. They can log in to the URL (external URL) which redirects to
...atlassian.net/servicedesk/customer/portal/1
...atlassian.net/servicedesk/customer/portal/2
...atlassian.net/servicedesk/customer/portal/3
etc
However they can use
a) smart search
b) "Help Desk" link on top of page
c) the master URL (...atlassian.net/servicedesk/customer/portals)
to switch between the portals.
It appears that this is solved by this thread :
https://answers.atlassian.com/questions/32970687
The key item is that under portal admin the client support portals require:
to be set to
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