The steps to merge two pieces of music into one piece of music with a 20-second interval in between are as follows:

1. Download and open the Format Factory converter on your computer, and click Advanced.

2. In the advanced option settings, click to open audio merging, as shown in the figure below.

3. In the audio merging interface, click Add File to select the two required songs to import, as shown in the figure below.

4. Click to intercept the clip and set the beginning, end, and 20-second interval of the merged song according to your needs.

5. After the above steps are completed, you can click Start to merge audio files in the menu, as shown in the figure below.

6. After the merge is completed, right-click the merged file and select Open output folder in the pop-up menu.

7. You can see the merged MP3 audio files, as shown in the figure below.