I can only answer the first part of the question - you can only install one instance of an add-on.
Whether a particular add-on provides connections to more than one system, that's up to the add-on. I don't know about the Zendesk add-on
Hello! My name is Genevieve Blanch, and I'm the Marketing Manager at RefinedWiki, creators of apps to give teams the tools to customize Atlassian platforms. Currently, 44% of the tech team at Re...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG