Batteries in series, typical stick welding voltage is ~27v. You might be able to light up on one battery, but you will quickly learn why it's called "stick" welding.

I wouldn't arc weld with any number of pairs of sunglasses, that was firmly tongue-in-cheek; but yes you are right, stacked glasses would be series.

Also, if you try this, before pulling the battery from the non-broken jeep, drive it to the top of a hill so you can bump start it later when the battery is too dead to turn the engine over.