The Honda EU generators use inverters to create the 60 cycle AC. This means that their 60 cycle output frequency is not dependent on engine speed, and they can run at lower, quieter speeds when less than maximum power is needed. Honda uses the "Eco-Throttle" do do this.
The other thing that the inverters can do is synchcronize their output with another EU generators. There are several ways to interconnect two EU generators. Before I go into that, let me explain how the power connectors are wired. The common 120VAC outlets are wired just like a house. The larger left prong is the common, and the smaller right one is hot. It is fed through a circuit breaker, which is 20A. There are also two 4mm banana plug jackson the front, and following convention, the right one is hot. The power for it does NOT, however, go through the circuit breaker, but comes off before it.
One way to interconnect two EU generators is to use the low-cost Honda EU1000 cable. This is a simple cable with "safety" shrouded banana plugs on each end. While it is fine for connecting two EU1000s, there are two major problems with using it with EU2000s. The first is that the output of an EU2000 is twice the output of an EU1000. I was concerned that Honda uses sufficient wire gauge to handle the output of one EU2000 on a cable designed for a generator of half that capacity. The second is that when this cable is used, the output is taken from the common 120VAC outlet, which is limited to 20A by the breaker. Two EU2000s can output MUCH more than this, and an air-conditioner will draw more than this on start.
Another way to interconnect two EU generators is to use two male ends of a heavy-duty outdoor extension cord, joined in an outlet box with a 30A plug. These plug into the common 120VAC outlets on each generator. The advantage to this is that it ensures the polarity is always correct, i.e. hot is connected to hot and common to common. The disdavantage is that if one of the plugs becomes unplugged, one of it's prongs is hot from the other generator, and it can electrocute a child or pet, not to mention unsuspecting adult, that would touch it.
The third way to interconnect two EU generators is to use one of the expensive, commercially available interconnect cables from Gen-Tran, which use the safety, shrouded banana plugs, or to make your own interconnect using them, for less than $50. Even if these become disconnected, there's no way to touch a hot prong, as shown in the picture below:
One of the things you have to watch for when using the banana plug connector solution is that you need to make sure that polarity isn't reversed. Connecting one generator's hot to the other's common, and vice versa, won't hurt the generators. One generator's inverter will just invert it's output to match the other generator. The problem is that the common 120VAC outlet on the flipped generator is now backwards. It's common lead is now hot, and any non-double-insulated tool plugged into it is hot with respect to the trailer.
The commercially available cables don't distinguish which of the two plugs for each generator should be hot. Both plugs are black (at least this is the way they used to be). If this is the route you will take, mark the plug that is connected to the right prong socket on the 30A receptacle. If you make your own cable, you can distinguish the hot wire, since it is black, and you can color code the plugs as well.
For my cable, I used a 30A "RV" or "Travel Trailer" outlet. You can put it in a single-gang or double-gang outlet box. I used a double-gang box to give extra width for stability.
The cable itself should be 12 average wire gauge (AWG) which has a current rating of 20A. This is more than the 16.6A maximum current capacity of the EU2000. I used heavy PVC cable, but the extra thickness of the insulation meant that I had to drill the back parts of the banana plugs for it to fit.
The shrounded banana plugs are from Mueller Electric. The red ones are PN BU-32603-2 and the black are BU-32603-0. I obtained mine from MCM Electronics where the red are PN 69-174 and black are PN 69-173. Here's a picture of the plug parts:
As mentioned earlier, I had to drill the back two pieces to fit over the thicker insulation, but only had to go 1/32" oversize. With silicone spray on the cable insulation, these parts slid but were still pretty tight. You also have to be quick on the soldering iron, or you'll melt the center plastic prong in the plug.
I did not put a 30A fuse or breaker in the 30A outlet box to protect the trailer cord. While two EU2000s are capable of 33.3A before shutting down, the 30A main breaker in the trailer is going to trip at much beyond 30A for any significant amount of time. You might also remember that there is no 30A protection if you are using a 50A male to 30A female adapter to get power from a 50A outlet in a campground. The 30A main breaker in the RV provides the protection here as well.
I did use a weatherproof cover, but there's no reason not to use an open plate on the 30A outlet box. You can's use the generators uncovered in the rain because the connectors on them are not weatherproof. For now, I'm just attaching the outlet box to one EU2000 handle with a yellow bungie cord. There are many ways to do this.
Hope this helps anyone who's thinking of making their own cable to interconnect two Honda EU2000s. Two of them put out a bit more than a 30A RV can use. Even when running 1500-1600W appliances, I use both, since the converter is also usually drawing current when charging the batteries. They're about 55 lbs each full of fuel, and lifting one of them onto the tall bed of a 4X4 truck is about as much as I'd want to try.