im trying to build a system in the antec nineteen hundred case. i want to know if there is any benefit to having 2x 2080s over having 2 or 3x crossbridged 1080s
This is a great question. Technically speaking, the GTX 1080 does not support 3 way SLI as 3-4 way SLI support ended with the GTX 900 series. You can still use 3-4 cards in DirectX 12 games using Explicit Multi-Adapter mode, but it depends entirely on how the game is programmed in order to determine if the game will even use the additional cards.
If the decision is between two SLI'd 2080's or two SLI'd 1080's, the 2080's would definitely be faster. One very important thing to consider with SLI is frametimes or micro-stutters. While SLI cards offer a higher framerate, a single card would offer a smoother, more consistent framerate due to the lack of SLI overhead. If it were my system, I'd choose a single RTX 2080 Ti over SLI 1080's or SLI 2080's, especially if you are not trying to push a high refresh rate 4k monitor.
The thing to keep in mind with SLI is that you will almost never achieve perfect scaling. You may use 100% of one card, and only 40-60% of another. If you are going for purely an aesthetic decision or pushing high refresh rates on a 4k monitor, then I'd go with SLI 2080's. If you are simply looking for the smoothest gaming experience at a normal resolution (1080p, 1440p) with high refresh rates, a single 2080 Ti would be perfect.