The Honda manual says that only two generators of the same size can be connected together. Mayberrys has experimented with this and come up with the suggestion to do unlike sizes together. For the time being, I'd stick with the Honda recommendation.
The paralleling cable is just a Y of the hots and a Y of the commons and a Y of the grounds all to a 30A receptacle. Some have done this with the male ends of extension cords joined together in a junction box, but if one plug comes out of a generator, the prongs will be hot from the other generator.
The Honda setup use shrouded banana plugs, but as someone else on the net discovered, there's no polarity on these and you can plug them in out of phase. The inverters in the generators are smart enough to compensate for this, but it results in you being able to be shocked on the neutral line. The Honda cable also terminates in a 30A twistlock receptacle, so you also need a dogbone to plug into it and give you a 30A RV female receptacle.
Probably the best solution is to make up your own paralleling cable with a junction box RV 30A receptacle and some 10 gauge wiring using these plugs
with red for the hot leads and black for the neutral.
WRT to EU1000 vs EU2000. If you want your generator to have a long life, you really shouldn't run it at more than 75-80% load. If you want your generator to run at lower rpms and be quiet, you want your usual load to be a fraction of its maximum. An EU2000 doesn't cost much more, or weigh that much more than an EU1000, and will be working much less hard, and at lower rpms, with the same load that would have an EU1000 working near maximum. Given the same load, the 1.0 gallon tank of the EU2000 would also outlast the 0.6 gallon tank of the EU1000.
[on edit] Keep in mind an 80% efficient converter charging two batteries at 20A each will be drawing about 5A at 120V... and if lights, etc are on with a 55-60A converter, this could max out the EU1000.
Finally, an EU2000 owner has scanned his manual and put it on line here.
Hope all this helps,