I don't think 2 farad is overkill, I use 1 farad per 500W RMS as wired. If he's got 750W just for the subs (assuming that was the continous power rating. I always post power in continous(RMS) values as that's what matters most.) then that amp alone would need 1.5 farads. So the extra .5 farads for up to 250W RMS for the other amp.. Sounds like he's right on target.
Even if the 750W was posted as max power, then yea, it would be overkill but more is never bad in caps. Less would be... but more doesn't hurt. What they do is basically try to act like a high-voltate, high-speed battery for the amps. When an amp need a burst of current, the alternator can't supply it fast enough and the battery is too slow, so the cap dumps it's stores in the chain and then re-loads off the alternator on the other cycle of the amp when it doens't need current. It doesn this hundreds or thousands of times per second. This effectively filters and conditions the power supply to the amp and reduces the power-drain on the other automotive electronics. This does NOT however fix the issue of an under-powered alternator. Think of a cap as a filter and stabalizer, not a power-source.