I think I see the problem, but without the error I can't be sure, could you post the error you are receiving?
Your import is almost correct, but you are probably seeing something wonky about how there's no transformer between DNAFASTAFormat and QIIME1DemuxFormat. Since your data is already from split_libraries_fastq.py you can just remove the --source-format option entirely (or set it to QIIME1DemuxFormat instead of DNAFASTAFormat).