You cannot use 1 command. You must setup socat to listen on db2 before you start the backup on db1. nc is a very old program. socat is the modern replacement.
db2:
cd /var/lib/mysql
socat - TCP-LISTEN:3306 | xbstream -vx
db1:
xtrabackup --backup --parallel 4 --galera-info --stream=xbstream | socat - TCP:db2:3306
After the backup stream finishes, you need to prepare the backup on db2, recreate the grastate.dat file using the GTID in xtrabackup_galera_info file.