cat *_1.fastq > concatenated_1.fastq
cat *_2.fastq > concatenated_2.fastq

should do it

When you say catenate do you mean you want to combine all forwards and all reverse reads into a single forward and a single reverse file, or do you want to combine R1 and R2 for each sample to get an interleaved fastq? Please try something, show the code, we will be happy to debug and guide you.

find . -type f -name "*_1.fastq" | while read line; do echo "pandaseq -F -f $line -r ${line/1.fastq/2.fastq} 1 > ${line/1.fastq/merged.fastq} 2 > ${line/1.fastq/merge_stats.txt}"; done

or

parallel --dry-run 'pandaseq -F -f {} -r {=s/_1/_2/=} 1 > {=s/_1.fastq//=}_merged.fastq 2 > {=s/_1.fastq//=}.merged_stats.txt' ::: *_1.fastq