2 1/2 years ago I bought the BERG fuel system for the Honda EU2000s I own. I posted a few questions about it here on the forum at that time.
The Hondas 2000s have fuel pumps. This attribute allows one to use an external fuel tank and run them through the night without worrying about refuelling.
The external tank system requires special fuel tank caps with dry lock hose connects, hoses with dry lock fittings, and an external tank. The fuel pump pulls through the generator's on board tank through the special cap via the hoses connected to a dip tube in the external tank. Kits are available, including a parallel kit. So if you are running two in parallel they both are fed from the single tank. By the way, the parallel kit will also run one generator alone as needed.
Unfortunately the fuel tank that came with the BERG system turned out to be of poor quality. Its cap leaked like a sieve and sloshed fuel in the back of my truck and took up too much space. The OEM replaced it with an equally poor product so I trashed it too and replaced it with a VP Racing Fuel Jug. The racing fuel jug holds more fuel, is vented effectively, and the square/tall fuel jug rides better and is more stable in the truck. The VP Racing fuel jug has a threaded pipe plug in the cap that allows you to do the plumbing for the hose connections to a flexible dip tube you can make with flexible tubing and an in-tank weighted fuel filter.
I boondock at an Indy Car race every year and we need the AC. So we have fully tested this system now and it works flawlessly with the upgraded fuel tank. I think I have indirectly sold numerous of these systems when people stopped by to to check it out and find out it is no fuss no muss system that runs for a very long time.
So my advice now is to skip the external tank portion of the BERG kit. Just buy the BERG caps and hoses. Buy a tank separately and I recommend the VP Racing Fuel Jug. The VP Racing jug's caps are O-ring sealed and the jugs themselves are of decent quality. You can buy spare caps and rig one up to work with BERG system. Here is a link to the VP jug:
VP Racing Fuel Jugs