Can one external voltage regulators run two alternators

[MECHMAN]

What matters, is the impedance of the rotor. The 91102R can handle 15 amps max rotor current. Generally speaking, no two alternators will pull more than 15 amps, but there are exceptions.

Also, if the alternators have two different power curves, and you run them both on the same regulator, the load will not be distributed evenly between the two alternators, which can cause one to work harder than the other.

Assuming that you are running one common voltage throughout the truck, the best bet would be to run two identical alternators with identical output curves on a single regulator, so that they are both synced with one voltage control dial. (that's what we would have sold you)