Thanks for your input, Pierre! Unfortunately, neither of the options works. Given three pairs of fastq files, using the word option outputs the first mate of the first pair of fastq and the first mate of the second pair instead of mate1 and mate2 of the first pair. See below my test case output:
make -n
flexbar -r Test/A1_1.fastq.gz -p Test/B1_1.fastq.gz
flexbar -r Test/A1_1.fastq.gz -p Test/B1_1.fastq.gz
flexbar -r Test/A1_1.fastq.gz -p Test/B1_1.fastq.gz
Edit: Nevermind! Finally found the way. I leave the fix here just in case :)
$(fastq_p1_trimmed) : $(fastq_p1)
flexbar -r $(word 1, $^) -p $(subst _1,_2,$(word 1, $^))
$(fastq_p2_trimmed) : $(fastq_p1_trimmed)
touch -c $@