Can I buy or make an adapter to hook 2 generators together to run a 220 volt welder?
No way. If you connect two ordinary generators together, they won't generate the exact same voltage, and even more importantly will be out of phase with each other. The result will be they'll spend all their energy fighting each other. Hopefully this would throw a breaker, but if not it could easily cause a fire.
There are generators that are designed with the ability to be paralleled (e.g. these Honda units, but that's unusual.