Matrix Nexus - How to Teach Back-Facing Tools?

Greetings. I have a problem with tools that are too long to measure with the tool eye. In other words, I have to "teach" the tools. I did this with the forward-facing tools, no problem but when I try to teach the Backwards-facing tool in the sub-spindle (in the same tool slot) it automatically goes back to the forward's tool.

So the question is, how do I teach the tool Z-length when I have two tools in the same slot? Thanks.

Edit:
To clarify what I'm trying to do. The tool marked in blue is the one I'm trying to teach.