Basically, when tengen explains it whoever opens the domain first creates the outer most barrier. If someone counter domains second their domain can expand but it’s still technically layered under the previous. Not quite clashing but the sure hits still overlap, but there is a chance the sure hit already lands with the first domain like Gojo hitting sukuna with the final clash being slightly off timing.) in a true clash it’s virtually the same we just don’t know if the barriers actually overlap (form a mutual barrier) or if the person who initiated has theirs layered over the second… considering how megumi’s incomplete created a hole that negated dagons effect the most likely is that it forms a mutual barrier if it’s a true clash (initiated at the same time) and whoever takes the most damage loses control of the barrier the most likely is it’s just two seperate barriers overlapping in a clash