Yup, that's the correct workaround. The idea is that if you don't have enough disk space to hold the backup, the process will fail. Excluding the other sites allows the backup to complete. You can install and set up Akeeba Backup on each site and create CRON jobs for each one of them (firing 10-20 minutes after each other) to automate the process without running out of disk space.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!