If you parallel two EU2000's to get roughly the same power as one Onan 4000, chances are the noise level will be about the same, and you have the additional chore of starting them individually.
I believe that running two generators in parallel won't necessarily be that much noisier. I'm basing that on a test done with one Honda generator in operation, two Hondas in operation, and one contractor generator about the same capacity as the two Honda's running in parallel.
Last spring, mechanical engineering students at the University of Alabama did some noise control tests with portable generators:
Noise and Directivity Tests: Honda EU2000i Generators in Single and Parallel Operation and Pincor 2000W Contractor Generator The pdf file for the test is located
here.
When I came across this, I was planning to buy a generator. The Kipor looked very attractive, but I kept running across sites on line where dealers had stopped carrying them -- one dealer's reason was lack of support for the dealers by the manufacturer.
I ended up buying the Honda 2000i. I've only used it so far to test it and to run it loaded about every month. However, it is very quiet, when running in the eco(nomy) mode. Originally, I wanted a larger unit with an electric start. However, the 49 or so lb weight means that I can put it away by myself. The recoil start is very smooth and easy. The generator didn't start right away out of the box. I was starting to get a little frustrated and then it started. Every start since then has been on the first or second pull.
We will probably buy a second generator to extend our ability to dry camp in some areas even when it is hot enough to make air conditioning a welcomed comfort.
One word of warning about buying a Honda Generator at other than a dealer. The Honda warranty requires information regarding the dearler that you bought it from. We bought ours on eBay and thus have no warranty. We also got it a couple hudred dollars cheaper than what out local dealer was selling it for on sale.