What’s your power supply model and/or wattage?
Normally when the cards are idle or you’re only using one of them (playing a game without SLI for example) then you may be fine, but with both cards enabled for Cycles it’s going to drive them both towards their maximum power usage. The 1080Ti is specced at 250 watts for an Nvidia stock version (potentially more if they’re overclocked) so you’re looking at up to 500 watts just for the two video cards, plus what the rest of the system needs.
Having the system reset would be the kind of thing you’d expect to see if the power is inadequate I think. If you’re watching it when it happens and there’s no BSOD, or other error but the system just hard resets then it’s probably more likely power than software. Also I’m not sure but I think some power supplies may need you to use specific connectors to balance the load across the multiple output rails.