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I missed this post as I was away on holiday. It depends on what you are wanting the component references to do. Eagle will not allow duplicate references so will re-number the PCBs when you try to combine them. If you don't mind that then simply make a copy of the Eagle files (just in case it doesn't do what you expected you will then have an original to go back to and try again). Open the first PCB, select it all and copy it (CUT in Eagle). Then open the second PCB and close the schematic. Paste the PCB to merge them.
If you want the component references to be preserved, run a ULP first. There are several possible ones, some aimed at panelizing. Have a look at copy-silk-screen.ulp. That copies the silk screen to user defined layers. So, even though the silk screen gets renumbered, you still have the original numbers available. You just need to make sure that you use the correct layers when generating the manufacturing files.