Combine voltage sources
If the generator does not have a 240 VAC output then it's unlikely you can combine two 120 VAC outputs to get 240 VAC.  The only way I can think of that is both safe and reasonably inexpensive (for some definitions of &quot;reasonably&quot;) is a transformer.
How much power do you need from this 240 VAC output?  If it's less than 1000 watts, or perhaps 2000 watts, then you could possibly pick up a step up transformer from a hardware store for under $100.  Any bigger than that and it's probably cheaper to buy a new generator with a proper 240 VAC output.
An example of such a transformer here:  https://www.homedepot.com/p/Power-Bright-2000-Watts-Step-Up-Down-Converter-110-120-Volt-220-240-Volt-Voltage-Transformer-VC2000W/302542246