As a small addition to the answers: NVLink (essentially) only fits the new RTX cards. The older cards use SLI technology to connect several cards. What looks (almost) the same at first glance is in fact a big difference. This is exactly why the connectors / PCBs on the top of the cards are arranged in such a way that you are not even tempted to connect them with either an NVLink or SLI bridge...

I have only ever used identical cards, I would recommend at least the same model type (e.g. 1080Ti).
In the meantime, however, I only use one card (2080Ti). The extra performance is simply too little or only really decisive for a few titles (e.g. Tomb Raider) and is also supported less and less because the effort is not worth it for the developers.

i also recommend two identical cards. if you really need more space, there is this to bridge the extra slot: ASUS ROG-NVLINK-4, but with your current setup with two different cards, I can't imagine that this nvlink is the right one for you.
good luck :)

The ASUS Rog-NVLINK-3 only fits the latest graphics cards. If you want something like this to work in the end, you have to use 2 identical cards. But honestly, better a "real" one than spending 3x money!
Greetings Cleaner