Hi Febri,
In single-thread mode (ie. `j=1' and/or `np=1'), the two FASTQ files would be read one at a time.
In multi-thread/process mode (ie. `j=2' and/or `np=2'), the two FASTQ files would be read in parallel.
Since reads are broken up into k-mers to generate a de Bruijn k-mer graph, the origin of the k-mers (ie. position within a read file, which read file, which file format, etc.) should not matter.
Hope that helps! :)
Ka Ming