40 Amps at 14 Volts is 0.75 hp output, so two 40 amp alternators produce 1.5 hp output. The alternator and belt drive inefficiency would raise that to maybe 3 hp engine power to drive them to full output. However you need to remember that a 4 hp Briggs only develops about 2.5 hp: the rest is Hollywood fiction, permitted by the remarkable test procedure the engine manufacturers use.
Remember that those car alternators will not produce any output unless energised by something like a battery across their output terminals. If they don't have built-in regulators they will also need a pair of those.
