You'll need to determine how much power the two Cpap machines draw and then match the generator with the correct sized output in order to get the smallest possible. I had a Honda 1000 watt generator back in the 80s, it was very quiet. About the same noise as a portable fan on medium spped.
Depending again on the draw of your Cpap machines you may be able to plug an inverter into a power outlet from your motorcycle battery or add a second battery for more power. Each of you could run your Cpap from your own motorcycle.
There are alternatives to the Cpap. Here's one Dr's website
http://drgreenburg.com/sleep-apnea/cpap-alternatives
I've also seen exercises that you can do to eliminate the need for Cpap by strengthening your throat muscles.
http://www.sleep-apnea-guide.com/sleep-apnea-exercises.html