Hi @mcboschman, thanks for your patience.. indeed interesting there was no issue in using both SDKs in a single application, though I would certainly be concerned about performance and reliability of timestamp data.
In any event, I managed to find a couple of sources for you in apply fixation filters to raw gaze data which should hopefully be useful. The code used is not necessarily c#, but I think you should be able to translate easily enough.
First link @ https://github.com/davebraze/FDBeye/wiki/Researcher-Contributed-Eye-Tracking-Tools has many different tools across different platforms.
and the Second @ https://es.mathworks.com/matlabcentral/fileexchange/56236-djangraw-gazevistoolbox is not designed for Tobii Trackers but the methodology is compatible.
Please let us know if these are of use to you, and if you manage to create a c# fixation filter kindly share your code for other to use! Thanks.