All that said, I've never thought about how you might get 240 out of multiple 120 volt generators. With DC, putting batteries in series adds the voltage together. Not sure that would work for 2 AC generators though???
The problem is trying to operate two generators in parallel. This presents load sharing difficulties. These generators aren't built with the regulators required to control load sharing because they were built as stand-alone generators for simple, single point power supply use.
If I were to try getting 240 VAC out of a 120 VAC generator, I'd probably just do it by connecting a step-up transformer to the generator. You could still get 120 VAC from the input side (the output of the generator itself), but the output of the step-up transformer would be 240 VAC.
However, one still has to be careful to remain within the capacity of the generator itself. You can't draw more power from the generator than it's designed to produce, no matter what you do with running the output through transformers. If you double the voltage through a step-up transformer, you cut the current available in half. If you have a 120 VAC generator rated at 2,500 Watts, then you have about 20.8 Amps available to power all your loads that run off of 120 VAC. If you want to power equipment at 240 VAC, then you'll only be able to draw 10.4 Amps max at that voltage. If you're powering both 120 VAC and 240 VAC, then you need to be sure you don't exceed your total power capacity between the two.
A transformer like the following would do it, but it would cost you about $200, give or take. Then you'd have to wire it up yourself.
https://www.hubbell-acmeelectric.com/ds-1716-datasheet
Remember...240 VAC loads are generally pretty big loads...and in an emergency response scenario, is it really necessary to power those kinds of loads? The two main household appliance loads requiring that voltage are electric ranges and electric dryers.
The vast majority of your electrical needs are 120 VAC. You need, as a minimum, enough power to run any life essential medical equipment you may have, your food storage appliances for perishables (refrigerator and freezer), and communications equipment. This is what should drive your base-model need for a generator.
From there, anything more really falls under the "nice-to-have" category. Air conditioning? A small 120 VAC window unit for those who really need it. Light usage should be bare minimum. Washing needs should be by hand, then hang dry instead of using the washer and dryer.
If you REALLY want all that other stuff...just bite the bullet and buy a generator capable of supporting all that.
