Yes the loss is due to hp restrictions. We factor in a 3hp loss for the operation of a generator.
The formula to figure what you can achieve is as follows gpm x psi / 1100= required hp for operation (gas engine). So if you deduct 3 hp from the 18 hp engine for the generator you have approx 15 hp to work with. Then you take the 5.2 gpm x 3000psi/ 1100= 14.18 hp required. (our specs for generator unit)
As for the dual unloader setup most machines come with an unloader so that really wouldn't be necessary and the market isn't really there for an unloader-less machine. The people that would want something like that, such as yourself, are already knowledgeable enough to build their own so... But you are correct, as long as the pressures of the two machines are equivalent you could link them that way if you wanted, just be sure you are using trap style unloaders for it to work.
We do have a unit like that already in our
dual pumper, 1 engine 2 pumps up to 20 gpm.