The Fractal line is often used by watercooling builds, so you could find several examples online as to what might work. You're looking at the following TDP loads based on typical base thermal design:
i9 9900 - 95watts at factory clocks/settings
2080Ti - 250-260watts (x2 = 500/520watts)
At factory clocks, you're looking at around 600 watts to cool in total, which power draw cannot = thermal load output 100%, so anywhere from 70-85% is more realistic, making this value around 450-510watts, although I like to lean towards the higher end. This could be handled by a single (thick) 360 radiator if you really wanted to. Otherwise, a single 360 for the GPUs would easily handle those cards and you could run a thick 240 or 360 for CPU, if...