A 3090 often goes for $800-900 on the used market in the US. Two of these would be $1800, and you get a much more versatile machine. However, the downside is also obvious, since your two 3090s can draw up to 800 W, and there's no chance you can carry it around with you. Overall, it's not that obvious where the actual value is, as it depends a lot on what you want to use it for.