It shouldn’t be too hard.
If you know where each planes folder is, then:
Rename the Beech99 sound folder.
Copy the kingair sound folder and paste in the Beech99 folder.
Rename that kingair sound folder, if necessary, to be the same as the original Beech one was.
Now you need to make a new layout.jsn file to replace the existing one in the outer Beech99 folder.
I use MSFS Layout generator to do that.
That should do it!