Let's give some generic answer to this, because the question lacks the necessary information about the control circuitry to give a dedicated answer for those two DG sets.
Judging from your data both devices are synchronous generators, enabling you to control two parameters: voltage and frequency by changing excitation and speed. For a successful synchronization of two generators (or a generator with a grid) these are the only prerequisites. Synchronization refers to just the moment you connect both DG sets. Successful means not tripping any safety mechanism instantly or damaging one of the DG sets.
There is no theoretical limit for synchronizing generators of different power, different current handling capability, very different rotational momentum or other differing factors. In fact you could synchronize an 1 kW DG set with an 1 MW DG set when the four sync conditions (phase sequence, frequency, voltage, phase) are met.
However synchronization is only half the work when you want to make use of more than one generator on a grid. The more difficult task is to enforce a useful sharing of real and reactive power.
Real power sharing can be achieved if
- the nominal frequency of at least one DG set can be controlled directly
- both DG sets have a governor with a well defined load dependend droop
- either both droops are the same over the full load range (e.g. 4%) or if one can be adapted.
Reactive power sharing can similarily be achieved if
- the nominal output voltage of at least one DG set can be controlled directly
- both DG sets have an excitation regulator with a defined droop dependend on reactive power output/input
- either both droops are the same or if one can be adapted.
Without the droops and setpoints for frequency and voltage correctly configured for a power sharing you may face several problems after a seemingly successful synchronization. A non comprehensive list of potential events:
- One generator running close to its nominal load while the other is nearly idle or even
- One generator taking up power instead of delivering it (very dangerous)
- In consequence of these conditions protection circuits of one or even both DG sets tripping
- One generator picking up the majority of reactive power when connecting inductive or capacitive loads to the grid
- Both generators exchanging reactive power even with no significant load connected to the grid
The last two effects are even more sneaky than an uneven distribution of real power. Winding losses grow with the square of the reactive power as the generator voltage is fixed, so it is crucial to keep the reactive power under control.
There's one more thing with big generators: They should have a damper winding to prevent oscillations between both sets. Consider the rotor inside the magnetic field as a mass connected with a spring to the outer world (grid). This mass-spring system has a certain resonance frequency which can be excited by external triggers. If more of these mass-spring systems are connected together the oscillation can increase when the resonance frequencies are tuned. This can lead to additional stress on all mechanic parts. Probably the bearings of the diesel are the most affected ones.
Having mentioned the various problems which can arise within a number of already synchronized DG sets it is obvious that the control options (nominal voltage/frequency and voltage/frequency droop) make no sense without the possibility to measure and display real and reactive power for both DG sets in summary and preferably for each phase, too.