Don,
I referenced NEC 630.11(B) (Group of Welders) Mainly because I couldn't find anything else even close. Upon reviewing it further and more careful I noticed it states group of welders not group of welder outlets. Based on that I would presume Article 630 would not prohibit such an installation. There doesn't seem to be a clear cut answer I can find in the Code Book.
As Scott mentioned, The OCPD would open in any event to provide safety. Beyond that it's a design issue.
Frank
