It can't be done, it's a limitation of the radio.

The Eclipse is quite a dated (but still quite functional) design that was conceived before split elevators became as common as they are now.

The Optic 6 has this capability -- but then you're a channel down.

You might be better off with a reversing Y-cable, but watch that you get one which allows you to set the end-points if you're not using Hitec programmable digitals.