I don't think you can.
What you'll need to do is create dashboards using filters that contain issues from both projects. With the right combination of gadgets on a dashboard, you can probably replicate most of the out-of-the-box reports.
If you're trying to replicate a particular report, let me know and I can probably suggest a couple gadgets to get you what you need.
There's also plenty of plugins out there that can help with this, but it sounds like you'd like to avoid that route. If not, I've had good luck creating custom reports with the PDF View plugin on JIRA Server.
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