Here's how to get either 110VAC 30AMPS or a 220VAC circuit from 2110VAC extension cords. (no O-scope required)
Take two beefy 12Ga extension cords and plug them into 2 receptacles in different rooms. Each cord can safely carry the current that it was designed to carry.
The other ends go into a large electrical box.
In the box there are two beefy 110Volt 20aMP receptacles and a 220Volt receptacle.
The 'weak' link is the 20 Amp receptacle that is under rated for 30-40 Amps. However, the welder is just a few feet away and I do not consider it unsafe.
In a 3 hole receptacle there is a wide slot on the left, neutral A small slot on the right, hot A round hole beneath, ground.
Using a multimeter on the 250 VAC range, measure the voltage between the 2 hots.
If it measures 220, you connect each hot to 2 opposite terminals of the220 receptacle and the neutral and the ground to the third terminal. You have a 220 VAC circuit. Plug your 220VAC welder in there.
If the multimeter measures 0 you have tapped the same side of the circuit breaker box. Flip one extension cord circuit breaker off. Measure both 110 receptacles. If both are 0, you must find another 110 receptacle from a different breaker..
If only one circuit went off when you flipped the breaker, you have it.
Just connect both receptacles in parallel and you can draw 2x Amps from your extension box. Plug you high draw 110 welder in.
The technique might be useful in reverse when connecting a generator to your home in an emergency if the generator is far from a 220 circuit. (I used the clothes dryer receptacle last year after hurricane Jeanne.)
Here's a commercial (pricey) box.
What is not obvious is why it isn't obvious to everyone.