Ger09,
What are the welding process(es) involved? I've never experienced any problems with (old school rectifiers) Miller 8-packs on a common ground system. Some welder's were using DCEP for SMAW while others were GTAW on DCEN.
I do not know if multiple, dissimilar processes on the same circuit would make a difference. Such as running GMAW-S and GTAW Hi-freq.
Excellent question and I am hoping those with a greater grasp of electrical theory chime in and agree or have a different opinion.

Great question, I always figured it was pure voodoo.  We ran similar setups, with guys welding Innershield on DCEN and air-arcing and stick welding on DCEP... never created any welding problems that I noticed, although I read somewhere that this can increase the risk to the welder due to elevated OCV potential - can't say if that is true or not.

http://www.aws.org/w/a/survey/standard.html?survey_start=z49_reqpdf

Excerpt 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.

Thanks all.
We're using FCAW for structural and GTAW for architectural.