Different Transformers Voltage

There will be circulating currents between the generators, and the voltage on the bus between the generators will be uniform except for minute variation from length and resistance.

...and in a properly designed and installed system, once the circulation currents rise to intolerable levels, (a) circuit protective device(s) will operate, disconnecting the relevant piece(s) of equipment from the supply and preventing the equipment and its cabling from incurring damage.

Why do you want to run mismatched transformers in parallel? Beside the probable circulating currents have you considered the maximum prospective fault current on the 400V board?

draw the circle diagrams and see the result . . . .

It's much simpler than circle diagrams, it's just the loop impedance with the difference in voltage applied across it. Basically you take the voltage difference (5 volts) and divide by the sum of the transformer impedances (not provided by OP but let's use X=0.1j), then 5.0/0.1j= 50 Amps. It may not seem like a lot until we consider that it gets added to the current in one of the windings, plus the extra losses are I^2 R, and 2,500 times any resistance can be a lot of kWhrs over the course of a year.

In my opinion, the generators-connected both with the 11 kV Busbar are synchronized-same frequency same voltage []

The problem is on 0.4 kV side and here it could be a difference leading to a circulating current between the two transformers.

IEC 60076-1 Table 1 - Tolerances, it states:

2. Voltage ratio at no load on principal tapping for a specified first pair of windings:

The lower of the following values:

a) ±0,5 % of declared ratio

b) ±1/10 of the actual percentage impedance on the principal tapping.

Let's say we are speaking about 11/0.4 = 27.5 [minimum 0.995, maximum 1.005] = 27.6375 to 27.3625 [11/27.6375 to 11/27.3625=0.39801 to 0.40201 that means maximum 4 V difference is allowable in order not to damage the transformers.