two identical elleptical plates each of mass M and MOI 'I' , about an axis passing through centre and perpendicular to the plane . Both plates are fixed concentrically in same plane as shown in figure . MOI of system about an axis Ab in the plane of plates and passing through centre is ????

For any two bodies if u need to find combined MOI about an axis u can simply add the separate MsOI of the bodies about that axis.

In this case it will be simply 2I.