EllipseGeometry is Geomtry too so you can combine like this
<CombinedGeometry GeometryCombineMode="Union">
<CombinedGeometry.Geometry1>
<EllipseGeometry RadiusX="70" RadiusY="30" Center="100,150" />
</CombinedGeometry.Geometry1>
<CombinedGeometry.Geometry2>
<EllipseGeometry RadiusX="70" RadiusY="30" Center="200,150" />
</CombinedGeometry.Geometry2>
</CombinedGeometry>
EDIT
To combine three or more EllipseGeometries you could use this mechanism
<CombinedGeometry GeometryCombineMode="Union">
<CombinedGeometry.Geometry1>
<CombinedGeometry GeometryCombineMode="Union">
<CombinedGeometry.Geometry1>
<EllipseGeometry RadiusX="70" RadiusY="30" Center="100,150" />
</CombinedGeometry.Geometry1>
<CombinedGeometry.Geometry2>
<EllipseGeometry RadiusX="70" RadiusY="30" Center="200,150" />
</CombinedGeometry.Geometry2>
</CombinedGeometry>
</CombinedGeometry.Geometry1>
<CombinedGeometry.Geometry2>
<EllipseGeometry RadiusX="70" RadiusY="30" Center="100,150" />
</CombinedGeometry.Geometry2>
</CombinedGeometry>
You can create a new class, that is derived from Geometry and have Geumetry[] in it and implement methods using above mechanism