We are to propose a solution for 6axis step&dir and 2 ENDAT encoders reading.

I would like to be sure the P849 is able to manage the two requests together.

Yes, EnDAT will work on any of the first 8 axes.

It does not work if there is another axis paired with the one on that connector. For example if axis 3 is Endat, then you cannot use axis 11.