touch a.fasta
touch b.fasta
touch c.fasta

find . -name "*.fasta" | parallel echo union -sequence {} -sformat fasta -outseq {}.union.fasta -osformat fasta -auto

find . -name "*.fasta" | xargs -L 1 -I '{}' echo union -sequence {} -sformat fasta -outseq {}.union.fasta -osformat fasta -auto