audio – FFmpeg. How to use afade on merged track in filter_complex

I have a video with two audio tracks. The following code raises the volume of the first audio track by 10dB and then combines the two tracks using -filter_complex. I can’t figure out how to create a simple 15sec afade on the final combined audio track.

ffmpeg -i "Input.mp4" -ss 00:03:35 -filter_complex "[0:a:0]volume=10dB[a0];[a0][0:a:1]amerge=inputs=2[out]" -map "[out]" Output.mp4

I’ve already tried the following:

ffmpeg -i "Input.mp4" -ss 00:03:35 -filter_complex "[0:a:0]volume=10dB[a0];[a0][0:a:1]amerge=inputs=2[out];[out]afade=t=in:ss=0:d=15[out]" -map "[out]" Output.mp4

And this:

ffmpeg -i "Input.mp4" -ss 00:03:35 -filter_complex "[0:a:0]volume=10dB[a0];[a0][0:a:1]amerge=inputs=2,afade=t=in:ss=0:d=15[out]" -map "[out]" Output.mp4