Oh wait, I have it all figured out.
Just go to the store and buy one of these, I am sure you can get a 240 VAC 50 Hz model. Only $650 USD. That is option #1.
Then we have option #2 which is just a tad more complicated. First we take the outputs of your two generators and convert the 240 VAC 50 Hz to 12 VAC 50 Hz. Now to be safe I like to overrate everything. I estimate you have 2 KW generators based on the 4 HP. So I figure a pair of 5 KVA transformers
like these units. They only cost about $900 USD each and you may want a truck to move them around. Then off each transformer build a nice full wave bridge using stud mount diodes and add a few large capacitors as generator power is sort of dirty. Now just parallel the two 12 VDC lines and hope the generator regulation is happy with that and they don't try to compete. So now we have 12 VDC and a pile of amps. Finally just buy a nice 6 KW (remember I like to overrate) inverter.
**broken link removed** which only cost about $600 USD. All said and done I figure a little under $3,000 USD including shipping (darn transformers are heavy) and you are done. Granted not as inexpensive as just buying a new larger generator but when all connected if it doesn't smoke it will be really, really cool and impressive.
Option #3 is run with what you have and divide the loads off each generator independently. The good news is during low demand you only need to run a single generator the bad news is you will need a small mountain of extension cords. As NorthGuy mentions, unless what you have are designed to be synched you cannot connect them in parallel. Generators designed to be synched are expensive and usually much larger than what you have. I would choose option #3. There is no practical or easy let alone inexpensive way to do this. Like I mentioned earlier looks good on paper but just won't work.
Now on a serious note, the 4 KW generator I mentioned earlier easily supported our home during a power outage. My wife's mother was totally invalid and as medical machines came into the house I had concerns about power outages. I could run every lamp in this house, all the medical equipment, home computers and the medical equipment and the furnace on about 3 KW. A big pain was it, unlike the new units, only had a 1.5 US Gallon gasoline tank. I needed to fuel it every two hours. It was also 8 HP pull start and all transfers were done manually. However, on a few occasions it was literally a life saver. Today we have a whole house fully automatic system with automatic transfer. Power grid drops out and in 8 seconds we are totally back to normal. It runs on natural gas so no fuel tank or gasoline storage needed. With 18 KW I can help my neighbors during long outages which is nice. Anyway, that little old 4 KW unit sure worked out great when it was needed.
What exactly is your goal? What do you plan to run? How often and how long?
Ron