Do not use 15amp to 30amp adapter , this can damage all kinds of stuff , the source 15amp outlet will burn out the socket trying to pull a 30amp load .
As for connecting 2 Honda / or other brands made to do this , you can buy 2 of the cheaper gen-sets , and then use a special combiner [ this is what Honda 1st came out with ] then later came out with one where all the wiring was on the inside of one unit for 30amps , with a 15amp connection going to a 2nd genset .
TWO things are happening with the combining of 2 gen-sets , having an outlet that can handle 30 amps [ supplied by 2 15amp souces ] ---- not over loading 15amp wiring - trying to pull 30 amps , the next thing some of the extra wiring is doing , is bring the 2 generator into the same phase [ match the AC wave forum ] .