What has mechanically syncing large generators got to do with the price of tea in China? I don’t think that was the question.
These small inverter generators are self-syncing.
I’ve connected Yamahas and Hondas. I’ve also connected various different size units.
There’s no different-brand issue.
Do you worry about plugging your equipment into different brand generator’s power?
Do you think your microwave or computer operate differently on Honda 60Hz 120VAC than they do on Yamaha 60Hz 120VAC?
No!
It works fine. The generators don’t know or care where the other source of 60Hz 120VAC is coming from, it is what it is.
Don’t waste an extra $100 on a Honda “Companion”. That’s marketing BS. It’s zero different other than having a 30A twist-lock outlet on its panel, which doesn’t help you anyway because it’s not an RV-30, so you’ll be using an adapter anyway.
You gain absolutely nothing.
What I’d do it if were you is get another Honda, a new EU2200.
It’ll cost less than the 2400, it weighs a lot less, and when you parallel it with your existing EU2000 you’ll have plenty of power.
Harbor freight sells a rather nice parallel box. I’ve always made my own but at their price if I needed one today I’d just buy theirs.
Parallel kit 
