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To merge two MP4 files without re-encoding, you'll need a tool that can concatenate the files directly, preserving the original encoding. Here's how you can do it using LosslessCut, a free and open-source video editor.

Using LosslessCut:
1. Download and install LosslessCut: LosslessCut is available for Windows, macOS, and Linux, and can be downloaded from their GitHub repository.
2. Open LosslessCut: Launch the application.
3. Add the MP4 files: Drag and drop the two MP4 files you want to merge into the LosslessCut window.
4. Merge: Click the "Merge" button.
5. Save the merged file: LosslessCut will create a new MP4 file with the combined content, without re-encoding, as long as the original files have compatible settings.