Step 1. Open both files in Sound Forge.
Step 2. Select all the data in one of the files (Edit > Select All, or use the keyboard shortcut CTRL+A).
Step 3. Copy the selection the the clipboard (Edit > Copy, or use the keyboard shortcut CTRL+C).
Step 4. Switch to the window for the other file. Put the cursor where you want the data inserted (for example, if the selection is the second half of a recording, put the cursor at the end of this part).
Step 5. Paste the selected data (Edit > Paste, or use the keyboard shortcut CTRL+V).
Voila!