"Cannot find a matching stream for unlabeled input pad 2 on filter Parsed_amerge_2" ffmpeg
Inside a filtergraph, each pad is associated with a single stream. And a specifier such as 0:a will match the first stream that qualifies. You need to specify two pads and fully specify the input streams.
[0:a:0][0:a:1]amerge=inputs=2[a]