Most geometry engine operations requires all geometries to be in the same spatial reference. As the error points to, that is not the case. Before performing any geometry engine operation, you could use the following code to bring geometry2 over to match the spatial reference of geometry1 (or vise-versa):
if (!geometry1.SpatialReference.IsEqual(geometry2.SpatialReference)) geometry2 = GeometryEngine.Project(geometry2, geometry1.SpatialReference);
The SceneView always returns coordinates in wgs84 lat/long.
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