Nice set up
...Although as mentioned before Lee, the polar mount bar extension that you have there may work but wasn't designed for coupling the two bars together. There are two small arms that come with this version of polar mount as standard, you can use either one depending on how you mount the jack. You shouldn't need to extend the original bar by joining together. A 12 inch jack will easily allow the mount to reach the polar E&W limits. I asked the questions and that was the answers I got.
If it works then all well and good