I LOVE the generator box! I'm going to make a larger copy of it for a pair of 2000s. Honda EUs are quiet - even quieter is even better, less dust is better, but the security aspect of anchoring the box down and locking it up is perhaps the best benefit.
Everyone should do the external fuel tank thing. http://ilynne.com/captaingoddammit/
I need to update that set of instructions, I'm not 100% happy with that exact method of plumbing the fuel line through the Honda gas cap.
One of the biggest benefits of the extra tank is that you can filter the fuel supply before it gets to the Honda at all, and you never have to open the Honda's tank.
Everyone should also make their own parallel operation cables, Honda's are a rip-off. The "parallel operation ports" on a Honda EU are simply wired directly to the regular power outlets on the front of the set. You can use nothing more than a short extension cord with the female end cut off and replaced with another male, then plug one end into each Honda. Better yet, buy two 12-gauge power-tool replacement cords from Home Depot and run both into a junction box onto which you've installed whatever outlets you want (I have an RV-style 30-amp outlet on mine so my camper plugs in without one of those
The main thing you need to know about operating two (or more) Hondas hooked together is start-up sequence. Connect both sets FIRST, THEN start either or both engines. Don't connect them while they are already running.
You can connect different size Honda EUs, I've run EU1000, EU2000 and EU3000s together in different match-ups and they all play nice together.
Once connected, you can start/stop either set anytime at all. You can run just one and just start the second one when you're gonna be using your microwave or whatever.
The only other caution in doing this is remember that with a double-male connector cord, if you unplug one end with the other end plugged into a running generator, you will have exposed, live terminals. Don't touch them and kill yourself.
A cool thing about running two is that even when you are only drawing a moderate amount of power (that just one set would handle), two units barely idling sound nicer than one unit throttled up, and with two you can usually get away with using Eco-Throttle mode even when you want to run power saws and other large start-up-power items.