What kind of artifacts? Most frequent problems with multi-gpu rigs are either when the cards just don't get properly recognized by the OS (device manager shows them with the exclamation mark, or doesn't show them at all), or they are recognized, but crash when you start mining with them. Graphic artifacts on the screen usually indicate some sort of hardware problem — either the card(s) is faulty, or it isn't supplied with what it needs to run correctly (too much overclocking/undervolting, problems with power supply etc). Though it's Vega cards in your case, so anything is possible with those..