Help Two 50 Amp Plugs to make 100 AMPS

So we sell food trucks with 50 amp 120/240 shore power connections and 10,000 watt 120/240v generators both connected to a transfer switch before the Breaker panel (all equipment is 120v). When customers need a 20,000 watt generator we do not sell them shore power connections because of the hassle and high cost. But we now see our competitors selling trucks with 20,000 watt generators and using two seperate 50 amp shore power plugs to achieve the higher amperage needed. My question is how can the use two 50 amp plugs to get 100 amps ?