Wiring Transformers in Series With a 240V Plug From a Generator?
If you have microwave oven transformers that are rated 120 volts and a generator with 120/240 volts, then it makes sense to connect one primary winding to one of the two hot terminals and the other to the second hot terminal and both windings to the neutral. That will load both sides of the transformer rather that putting all of the load on one side.

You will need to replace the high-voltage secondary winding of each transformer with a low-voltage winding that you wind according to instructions that you find on the internet. You can connect the two secondary windings in parallel. This scheme will provide twice the current you would have by using just one transformer.

That being said, asking this question here indicates that you are not very well qualified to undertake this project. You can not expect to learn enough here to do this safely. You should study some more about basic electrical circuits, transformers etc. You should also look at as many plans for microwave oven transformer projects as you can find and make sure that you understand them.

After looking at another answer that you have. I agree that connecting the secondaries in parallel is better. Double the size of the secondary wire compared to what is recommended for a welder using a single transformer and use half as many turns. That will give you the current increase with series windings.