Use
ffmpeg -loop 1 -t 67 -i background.png -loop 1 -t 6.5 image1.png
-filter_complex "[0:v]scale=640x720,setsar=1/1[v0];
[1:v]scale=640x720,setsar=1/1,setpts=PTS+46/TB[v1];
[v0][v1]overlay=shortest=1:x='max(0,640-(t-46)*100))':enable='gte(t,46)'"
-y output.mp4