Not sure I understand your method.  I would

Spline Modeler, create a 4 node square and center the section (Don't use the square tool, freehand it.  elongate it on the y axis, choose the first section and create a second section.  go to the 2nd. section and move all 4 nodes to the center.  Don't try and do this with extrusion or it may have issues.  Make sure the first section is set to shape-to-shape.  That should work.  You can adjust size and proportions by manipulating the first section only using the symmetrical extrusion envelope.  Keep in mind that sometimes a test render or the modeling or assembly room may not display it properly so do a semi final test render from the render room to get a realistic idea of what it will look like.