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Definitely add a second generator, this will give some redundancy if either is down for maintenance or other problems, you can still pump some water. Yes, you will need to split the loads, the cost for that should be minor compared to the cost of the generator. Once you have the pump motor started it will not need the extra starting current until it is turned off and restarted, so you don't need to size genset based on starting all at once, that just drives up cost of generator. So Daniel's answer is good. Second since generators are made in standard sizes, when you figure what the pump split you want to put on each genset (mix of submersibles and horizontal pumps in the same piping path - you want to be able to deliver water even if only one genset is running) use the 70% loading and buy the next larger standard available size unit (with allowances for high ambient temperature and altitude).
Third, watch where the crankcase oil vent line is located. When I was with a telecom company we had issues in Iraq with crankcase vent line blowing oil mist where air flow took it into the radiator which then clogged with the fine dust sticking to the oil on the radiator fins. This caused the genset overheating since air flow through fins was limited. Yes, wet stacking would be a problem if the units were loaded UNDER about 30% but I don't see that as much of a risk in your case. BUY and install fuel/water separators and keep spare fuel (and oil filters) on hand and sealed. Most generator engine manufacturers allow a limited mix of waste engine oil back into diesel fuel. Check on that and you can solve the problem of getting rid of waste engine oil, you probably will need to filter it. If you can easily split the loads don't bother paralleling the generators. Unless you can match the winding pitch of the existing unit (often hard to find out what it is) they will have cross current problems if you try to parallel them. Save the headaches! Make sure folks check and drain any water in those fuel/water separators regularly.
It can be a wise option to buy the engine set with dual oil filter banks and valves so one bank can be switched out of service to allow for then to be serviced, while the second bank is filtering engine oil, this reduces engine downtime and get more water pumped.