MacPro late 2013 Fire Pro D500 video card

Hi,

I have an MacPro6.1 late 2013, A1481configured with 3.5GHz, 6 Core Xenon E5 CPU, 128GbRAM, 8Tb SSD and 2AMD Raedon HD - FirePro D500 video cards. I am currently using macOS Catalina V 10.15.7.

I purchased this computer to take full advantage of its dual powerful GPUs to run a single software, X-Plane 11 flight simulator. Unfortunately, it will not perform as expected. Using a single 4K monitor the frame rates are very low even with low simulator settings. Activity monitor indicates that only one of the two video cards has activity, and this card is maxed out, outputting somewhere in the range of 9-14FPS on medium simulator settings. Running a 1080p monitors is actually the same. Is this normal, is there a way to "engage" the second FirePro card via settings? And if it is possible where can I find instructions. I contacted AMD and was told that all upgrades and settings are released by Apple along with the OS upgrades.

The 6 Core CPU has o issues running the simulator at full capacity. According to the Activity Monitor It never goes above 50% and uses no more than 3 cores. The bottle neck seems to be the GPU. Any suggestions are welcome.

Thank you