Voltage drop

A 65 amp alternator rating only means whatever the manufacturer wants it to mean.
In other words its a label, or viewed cynically, its a marketing number.
Usually the rating is something close to maximum output, which only occurs when the alternator is turning fast enough.
Whether this maximum is measured with the output regulated to 12 Volts. 13.5 Volts or 15 Volts is another thing only known by the manufacturer.
An alternator that can produce 65 amps at 15 Volts is producing 975 Watts vs one that can maxes 65 Watts at 12 Volts is only producing 780 Watts.
What we, the users, really care about is about is current available in the 14 to 15 Volt range.

If your fuel pumps are both running at idle, they are sucking a large amount of the alternator's capacity. In fact they are probably maxing it out, especially when the battery wants recharging. Alternator capacity at idle rpms is relatively low.
Depending on how the pumps have been wired in, pulling 26 to 30 amps may also be too much.