I am looking to build a high end design computer that can also run games on max settings. My only issue is would it bet better to equip the system with a single rtx 2080 or run dual gtx 1070 ti's through SLI?
Depends on what you wonna do with the system. Usually both single GPUs are capable for 4k gaming and as well usually I prefer a stronger single GPU over SLI due to known compatability issues and often leaking full support in games.
Looking at the current GPU prices I would invest my money either into a 1070 ti or 2080 ti as RTX is still too expensive.
Hope this helps to give you some guidance.