@regisb, It's a little hard to picture what you're describing - you may need to provide a sketch. Do you mean two lighthouses mounted on a vertical plane at different heights to form a tracking volume that is longer in the Z axis than X and Y?
To reach Viveport's customer service team, email firstname.lastname@example.org
To connect with a live support agent for hardware support, go to www.vive.com/support, click on "contact us" in the top righthand corner, and then opt to speak with a Live Chat agent David R
Technical Specialist - San Francisco, CA; Monday-Friday