Hi Eduard,
You can use the same ground plane for both.
As an example, you can see that we use the same ground for both devices in our FRDM-FXS-MULTI in the following link: http://cache.freescale.com/files/sensors/doc/support_info/FRDM-FXS-MULTI-B.pdf
There is an application note with some layout recommendations for PCBs using Magnetometer sensors that I think could be useful for you in the following link: http://cache.freescale.com/files/sensors/doc/app_note/AN4247.pdf
Have a great day,
Jose Reyes
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