Sometimes you want to merge two projects together for some reasons - like support projects for two clients that have just made a merger.
The easiest way to merge two helpdesk project into one, is to transfer issues from one project to the other. I'd suggest starting to transfer a single issue or two, just to check that they transfer well (without lost data).
When transfering the rest of the issues, I'll suggest activating the batch update mode in the issues list - click the cogwheel icon next to the export icon at top right. Then you can check all issues and transfer them.
Remember to check "Add a user to the project if she isn't already added in the project" when transfering issues, so all related users are carried over to the target project.
Warning: If you have any issue rules that trigger on issues being transfered, you'll want to disable them for the duration of the move.
Note: Whether the issues "transfer well", is dependent on the projects using the same issue field configuration (IFC) or not. If they use the same configuration, there shouldn't be any problems.
If they have different IFC's, some issue fields may not transfer over, unless you add them to the IFC of the target project before transfering any issues. You manage the IFC's in General -Settings - Issue configuration - Issue fields.
When you've transfered the issues and users to a single project, you may also need to inform your end users/clients from the defunct project that they should use a new support center with a new support center URL and support email address (the ones of the merged project).