Syncronization of Two Alternators

Not necessarily. I reproduce the following text from the famous IEEE GREEN Book:

Where multiple generators are solidly grounded but have switches in the neutral, there has sometimes been the practice of grounding only one of the several generators in parallel to limit ground-fault current duty or circulating thirdharmonic current. This will increase the fault-current duty in the grounded generator above that for which it would customarily be rated. A chart showing this difference appears in the Westinghouse Transmission and Distribution Reference Book. The ability to switch neutrals appears to invite operational errors that could affect integrity of grounding, allowing overvoltage on four-wire loads, which would result in failure to meet criteria for effective grounding or acceptable reactive grounding and thus would possibly violate the NEC.

Dear friend,

if the alternators neutral is grounded through the NGR, better open one neutral otherwise due to circulating current the NGR will get more heat.

Look into these previous threads, they may be helpful:

http://cr4.globalspec.com/thread/36055

http://cr4.globalspec.com/thread/50263

- MS

With modern alternators wound to 2/3rd Pitch, the problem of (Triplen Harmonic) Circulating currents is overcome. So, there need not be any apprehension of circulating currents even when the generators are earthed thro NGRs.