Same filter in different chains
Apply the subtitles filter before the split. Note that subtitles will be, proportionally, the same size in each output stream.

-filter_complex "[0:v]subtitles=sub.srt:force_style='Fontsize=18,MarginV=15',split=2[v480][v720];
[v480]scale=858x480[o480];[v720]scale=1280x720[o720]"