Create one split screen video, separate audio streams and a thumbnail from the resultant split screen video - all in one pass
Combined command:

ffmpeg -i input0.mov -i input1.mov -filter_complex "[0]scale=640x360[v0];[1]scale=640x360[v1];[v0][v1]hstack,split[video][image]" -map "[video]" output.mp4 -map 0:a audio0.mp3 -map 1:a audio1.mp3 -map "[image]" -frames:v 1 image.jpg


For a 4 videos into a 2x2 layout:

ffmpeg -i input0.mov -i input1.mov -i input2.mov -i input3.mov -filter_complex "[0]scale=640x360[v0];[1]scale=640x360[v1];[2]scale=640x360[v2];[3]scale=640x360[v3];[v0][v1][v2][v3]xstack=inputs=4:layout=0_0|w0_0|0_h0|w0_h0,split[video][image]" -map "[video]" output.mp4 -map 0:a audio0.mp3 -map 1:a audio1.mp3 -map 2:a audio2.mp3 -map 3:a audio3.mp3 -map "[image]" -frames:v 1 image.jpg