Import the first audio file into the program by selecting the "File" menu and selecting the "Import" or "Import Audio" command, depending on your program. A new audio file will be created on a single track.
Import the second audio file into the same program. This will create an instance of the audio file beneath the original one on a second track. If you were to press "Play" at this point, both audio files would play simultaneously.
Double-click the wave of the second audio file track. Open the "Edit" menu and select "Cut."
Place your cursor at the point in the first audio track where the audio file ends. Open the "Edit" menu and then click "Paste."
Export your audio file using the "Export" command in the File menu.