If it is listed for multiple wires, you can put 2 or more under the screw. For example, Cutler Hammer CH breakers are listed for two wires of the same size under the plate of some of the smaller breakers. and commercial grade receptacles and some switches that use pressure plates can accept two of the same size. Those arent screws tho, just terminals that accept 2+ identical size/material wires.
2 wires under one screw is a commonly found violation in older work. I've seen it so much I have to think it wasnt a Code issue 40+ years ago, or no inspector cared about it.
With a disco or a panel, 30A in your case, you can pigtail another wire to the existing two and run the pigtail into the disco lug/screw, tho my gut tells me there is something wonky with 2 #10s in one lug in the first place.
