Perhaps others will not struggle as I did attempting to find the correct photos with board Masks and Pinouts, and have had no issues connecting the v2.9 board to the PandaCAN board. Photos of various boards are scattered for the 2.0, 2.5, and 2.8 board, and a few of the v2.9 board, but none show the two wired together!
If you figured it out right away, this post isn't for you.
For those still attempting to wire the PandaCAN to the PandaPi v2.9 board using the CAN Transceiver that comes with the CAN board, this is for you.
After hours of going through images and descriptions, I finally hit on the fact that the PandaPi v2.9 has the CAN Tranceivers on board, and the tranceiver wires that come with the kit are not used.
You simply need four wires between the two boards; 12-24 VDC, Ground for Power, CANB High, and Low.