@Veronica > Just to be 100% sure you're using the correct CDJ Aggregator, could you please go to this link HERE, continue to the CDJ-350 Firmware & Drivers page, and download the 'Mac OS Tool'.
Once you've installed it on your computer, ensure both your CDJ's are connected to your Mac directly via USB cables, and power them on. Then open the CDJ-Aggregator tool and please follow the steps outlined in this Instruction Manual I have attached.
Please follow all the steps exactly as instructed.
It will ensure you create an aggregate device, and that you select the same aggregate device you just created when you're in the Traktor Preferences.
Please let me know how it goes and if you have any more questions. :)