You will need to create an aggregate device
For best results, ensure that you set one box as the clock master and the others as slaves. On the USB Audio control app, ensure that the Slave boxes are sync’d to the master with a WCK or DI connection.
If you do this, you should uncheck the ‘Drift Correction’ box.
Multiple Titan and Atlas units can also be connected to a Pro Tools | HDX card, Pro Tools | HD Native PCIe card or Pro Tools | HD Native Thunderbolt unit, and they will to operate in sample sync in Pro Tools | HD software. These interface devices work as Core Audio devices that can be used with software other than Pro Tools | HD.
Multiple Titan and Atlas units fitted with MDIO-Dante modules will operate in sample sync when connected to a Dante system and synchronised correctly. Dante Virtual Soundcard or a Dante PCIe card both can be used as the host interface in system like this.