http://www.aws.org/w/a/survey/standard.html?survey_start=z49_reqpdfExcerpt from ANSI Z49.1 2012"E11.3.6.3 When operations on one structure
involve several welding machines, the dc welding
process requirements may require the use of both
polarities, or supply circuit limitations for ac welding
may require distribution of machines among the phases
of the supply circuit. No-load voltages between electrode
holders will be two times normal in dc or 1, 1.41, 1.73, or
two times normal on ac machines. Similar voltage
differences will exist if both ac and dc welding are done
on the same structure."
My post here is more of a safety consern rather than a performance issue.