Tente
ffmpeg -re -v info -i "rtmp://localhost/live/stream live=1" \
-filter_complex "[0:v]split[a][b];[a]scale=540:-1[oa];[b]scale=720:-1[ob]" \
-map [oa] -c:v libx264 -b:v 500k -preset faster -copyts \
-map 0:a -c:a aac -b:a 64k -ac 2 -flags +global_header \
-f flv "rtmp://localhost/live/500k live=1" \
-map [ob] -c:v libx264 -b:v 800k -preset faster -copyts \
-map 0:a -c:a aac -b:a 64k -ac 2 -flags +global_header \
-f flv "rtmp://localhost/live/800k live=1"