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Open the circuit you want to copy. (If you have made both, schematic and board already open the schematic)
Group the entire circuit and cut it to the paste buffer. Close the file and open the one you want to add the previously stored circuit. Use "Paste" and place the circuit where it fits into the frame or close to it. Then rearrange the circuit to fit into the frame. Copying two schematics into one you won't have to bother about part numbers. They will automatically be generated to avoid multiple equal part numbers.
If you just want to combine two existing boards proceed as before, but load the board file instead of the schematic. Copy the entire board including the dimension layer and board outline.
Be aware that merging two boards the forward/backward annotation won't work anymore. Don't save the merged schematic or board files with the original name! Your original design will get lost that way.