I would (and have) put as many welder outlets where convenient for the users.
I would talk to the customer and be up front with them.
I can do it for this lower price and it may trip, or for more money I can separate them and prevent any possibility of a trip.
Also it is worth taking a look at the "Electric Welders" Article in the NEC (630 in 2002). The rules are much different than the normal rules, you may use smaller circuits than you expect depending on the welders duty cycle.
[This message has been edited by iwire (edited 11-17-2006).]
