did you try locking the audio 1 track before dragging your other wav file over to audio 2 ?
No. I didn't do it until you said it.
I just now did it and it is still not working. It's still being put in as an overlay, indicated by the history panel. I locked it, so the diagonal lines are over the audio track, and it still merges. If it isn't merging, I'm not seeing it anywhere else.
You are trying to drop a mono clip into a stero track. You cant do that.
Create a mono audio track, or drop the audio over the top of the timline and Pr will create one for you.
To tell how an Audio Track is defined, look in the Audio Track Header. There will be a single "speaker" icon for mono, a double "speaker" icon for stereo and a "5.1" icon for DD 5.1 SS Audio.
If you do not have the proper Audio Track available, you can do one of two things: create the necessary Audio Track (per Curt's suggestion), or just drag your Clip below the Master Audio Track, and the necessary Audio Track type will be created for you.
Good luck, and glad that you got it sorted,