Here's a good article
that explains the ground-neutral bonding issue with portable generators. The upshot is to bond the ground and neutral together at the generator. The article explains a simple plug you can make for this purpose.
There's also an interesting comment regarding running two Honda generators in parallel:
Mike Sokol says: January 13, 2014 at 7:53 am, I’ve had several readers use the G-N bonding plug on Paralleled Honda generators. One was done using an aftermarket parallel kit, while a few of them had one of the Honda companion generators which doesn’t require an external kit. These all worked well, with the only slight exception being the external parallel kit version when the interconnecting “parallel” cables were accidentally phase reversed. That phase only caused a 3-light tester to indicated H-N swap on the outlets in the RV, but otherwise worked. So I recommend that you use the G-N bonding plug on the “second” generator feeding your shore power plug, plus make sure you plainly mark the parallel interconnecting cables so it’s clearly visible how they plug into the generators. Apparently those parallel interconnect cables only have a small colored stripe to indicate the connection which is easy to get mixed up.
This may explain your reverse polarity fault.