Vinyl Window Issue

Most of my windows are high end, vinyl replacement windows. The face of the upper sash sits almost flush against the lower sash where they meet. There is no horizontal, flat surface on the upper sash where it meets the lower sash.

This prevents me from installing the magnet on one sash and the sensor on the other. In order to cover these windows, I have to use two sensor sets. I mount one set at the bottom and one set at the top. The magnet is attached to the sash and the sensor is installed on the window frame. I'm considering upgrading to SSIII, but I prefer not to have to double up the sensors on each window. Has anyone else encountered this and if so, were you able to come up with a way to avoid using two sensors per window?