Sorry dumb question :)

There are no dumb questions...no worries...lol.

The Asus Strix Z390-H motherboard does support NVidia SLI/N-Sync so yes, you certainly could add another RTX 2080 later for 2X N-Sync. Some games will support SLI/N-Sync, while others may not. It doesn't hurt anything in unsupported games, just cant take advantage of the benefits. For most folks the decider will be the cost of dual cards, and right now the performance benefits of SLI/N-Sync don't really justify the double GPU cost.   That said, it's up to each individual gamer to decide if it's easier to go SLI to get a boost in compatible games performance, or to replace with a single GPU with new tech.   

One added consideration for going SLI/N-Sync 2X RTX 2080 is the PSU requirement. If you didn't plan on possibly going SLI when you bought the rig, then you may not have a PSU with sufficient power for 2X SLI RTX 2080. You will probably want a 1,000 watt or 1200 watt PSU to maintain sufficient power headroom, especially if Overclocking.   Changing out a PSU is not very difficult if the current PSU is modular cabled.

The current NVidia RTX line is performing so well that there is no real need to upgrade yet for most folks.   Despite new GPU tweaks coming soon, I don't see that changing much over the next year... Current RTX GPU's will still dominate the market.   NVidia will certainly come out with their newest line of GPU's mid-year, which may lower prices on the current RTX line a bit, but those new NVidia cards will likely be priced painfully higher.   You will have to weigh the plus and minus of your GPU choices then.   Keep in mind that all new GPU's, when they first emerge, will carry a certain risk due to the as-yet unknown track record of the hardware in the wild.

Hope that helps as you consider your options....    

Yup!   

You can find all the info I think you'll need here:

https://www.techpowerup.com/review/nvidia-geforce-rtx-2080-ti-sli-rtx-2080-sli-nvlink/9.html

The main take-homes are that NVLink is a great tech which tends to optimize the memory usage of both cards. It is next to useless at 1080p and 1440p, often not out-performing a single card. "Our results show that you really do not need SLI for 1440p or 1080p regardless of the refresh rate."

At 4K, it is completely dependent upon the game itself. Out of 23 very well-know titles, "11 titles showed negative or no performance scaling when switching to SLI."

It's really a mixed review as you will see, but the bottom line for you with a 2080 is stated thusly:

"So the burning question: should you spend $1,680 on RTX 2080 SLI? Absolutely not. Averaging all our tests, RTX 2080 SLI is within single-digit percentage performance with RTX 2080 Ti." It would seem "in a couple of years", the 2080 equivalent would be vastly stronger (and more relevant) than 2x 2080 in NVLink.

As stated, many games will never scale with NVLink, and the number that can seems bound to shrink as the single cards continue to grow more powerful. This is simply because devs will not see any need to waste time (which =$) programming for such a tiny slice of the overall gaming market.