[FFmpeg-user] toggle between two streams

[Michael Koch]

I have two video streams and want to toggle between them in 1 second 
intervals. I did try this command:

ffmpeg -i in1.mp4 -i in2.mp4 -lavfi streamselect=map='mod(t,2)' -t 30 
out.mp4

This doesn't work because streamselect doesn't accept expressions.
Who has an idea for a workaround?

Thanks,
Michael