[Service]
ExecStart=/home/myself/logtest/runlogtest

#!/bin/bash

...
/home/myself/logtest/app1 &
/home/myself/logtest/app2 &
...

journalctl -f | grep runlogtest

Aug 07 14:22:04 localhost.localdomain runlogtest[8742]: B
Aug 07 14:22:07 localhost.localdomain runlogtest[8742]: A
Aug 07 14:22:07 localhost.localdomain runlogtest[8742]: B
Aug 07 14:22:10 localhost.localdomain runlogtest[8742]: B
Aug 07 14:22:11 localhost.localdomain runlogtest[8742]: A
Aug 07 14:22:13 localhost.localdomain runlogtest[8742]: B
Aug 07 14:22:15 localhost.localdomain runlogtest[8742]: A

systemd-cat -t app1 /home/myself/logtest/app1 &
systemd-cat -t app2 /home/myself/logtest/app2 &

/home/myself/logtest/app1 |& systemd-cat -t app1 &