"If each of the (2) 110v circuits are 15a, does that give a 30a rating after the 2 legs are combined through the 220v receptacle, or does it remain 15a?"

It remains 15 A at 240 V.

Unless you have a small welder, not a great idea since it will overload the 15 A circuits (and probably trip them off as soon as you try to strike an arc).

They make welders that run on standard 120V. Sorry, you probably didn't want to hear that.

I used to sketch out jury-rigs such as yours... luckily someone wiser was always around to say "put down the pencil, back away slowly..."... lol.

The welder is in excess of the 15a that the standard 110 circuit would provide. I know of the 110v portable mig welders, but the higher voltage ones can weld heavier thicknesses of metal. If the (2) 110v legs combined only offer the amps rated to a single leg, then I guess the Idea is dead in the water....Hypothetically speaking though, does the rest of the idea hold water? Is there any other reason that you can't link the two 110v outlets (fed by the (2) separate legs in a circuit panel) to make a 220v portable outlet?

It's a bad idea. Buy a 120v welder or a gas powered welder and long cables.

"I used to sketch out jury-rigs such as yours... luckily someone wiser was always around to say "put down the pencil, back away slowly..."... lol."

It can actually work if you only need 15 A at 240 V.

When doing refrigeration work there is often not easily available 120 V to run a vacuum pump (or recovery equipment) since the requirement for a 120 V receptacle near a compressor/condenser is relatively new.

Running from one leg to ground at the outside disconnect is pretty common.

all of the conductors of a circuit must be contained in the same conduit or cable etc...

A good way to get someone killed. When the welder is "on", removing one of the 120V plugs may yield a nice suicide cord, the plug blades will be hot.

I solved part of a similar problem by making adapter pigtails with common 240V range/dryer/AC plugs and a welder receptacle. Still probably not 100% "to code", but not nearly as dangerous as a pair of 120V plugs.

"all of the conductors of a circuit must be contained in the same conduit or cable etc..."

That does not say anything about utilization equipment that is cord and plug connected.

There are numerous ways to rig a device like this that prevent exposure to a male plug with any voltage present.

Why not consider purchasing a 120v-240v transformer? A 5000 watt unit will run around $150 and you could use it on any job site. One example is on link below.

Here is a link that might be useful: 5000 Watts Step Up - Step Down Transformer