nullsrc=size=200x100 [background];
[0:v] setpts=PTS-STARTPTS, scale=100x100 [left];
[1:v] setpts=PTS-STARTPTS, scale=100x100 [right];
[background][left]       overlay=shortest=1       [background+left];
[background+left][right] overlay=shortest=1:x=100 [left+right]

ffmpeg()
.input("vid1.mp4")
.input("vid2.mp4")
.complexFilter([
    "[0:v]scale=300:300[vid1]",
    "[1:v]scale=300:300[vid2]",
    {
        filter:"pad",options:{w:"600",h:"300"},
        inputs:"vid1",outputs:"padded"
    },
    {
        filter:"overlay", options:{x:"300",y:"0"},
        inputs:["padded","vid2"],outputs:"output"
    },
    "[0:a]volume=1[audio0]",
    "[1:a]volume=0.1[audio1]",
    {
        filter:"amerge", options:{},
        inputs:["audio0","audio1"], outputs:"audio"
    }
])