Yes, as the model is designed to take up to 4 graphics cards. However, one would need to pick the brand of 2080 Ti carefully, to ensure they would fit.
Edit: Found this image showing two 1080 Ti cards inside a PO9-900:
Now 1080 Ti reference cards are the same length as 2080 Ti reference cards. Using the DDR4 RAM modules as a length reference of 5.25 inches, those cards are roughly 10.5 inches in length - so these are definitely reference size cards.
But if you look carefully, you can see that there seems to be a small bracket at the right hand end of each graphics card - these look like cable guides of some kind, but it's unclear if they're removable or not. They're the only thing that would appear to prevent using a 2080 Ti longer than the reference size.