-codec:s:0:1 is incorrect. If you use
-codec:s:0 then ffmpeg will use the stated codec for the first subtitle stream being passed to the output, if you use
-codec:s:1 then it will use it for the second subtitle stream, etc.
You can also use
-codec:s to select all output subtitle streams, or
-codec:2 to select the third output stream, regardless of what it is.
You are probably confused because the
-map option behaves in a different way - there, you have to select which input the selected stream comes from. (so,
-map 0:s:0 would take the first subtitle stream from the first input, and feed it to the output). However,
-map is for selecting which streams you want to take from the inputs; whereas most of the other options that use stream mapping are for use on the streams after they have been selected (so there's no need to specify which input file they're from), as they are passed to the output.