Is it possible to connect two power transformer in parallel but it is not the same rating ?

You can combine two transformers in parallel if  they have the same power. the same short-circuit currents and the same group of compounds winding only.

No............................

No if you are using EHV or MV Transformers used to supply to the grid

Even if you are using both as a distribution transformer at receiving end the TTR Ratio will cause a problem as it’s not an efficient option as the voltage of both the transformer may not match fully resulting in splitting of phase angle or the cos ф (phi) angle which will eventually unnecessarily heat up both transformer or even cause failure.

My suggestion is to sell both & buy a used or new one as per your requirement     

You can connect transformers in parallel connection as long as they have similar shortcircuit resistance.The power of them could be different.You should take into consideration the index connection between winding, because must be similar or adaptable among them. The voltage relation between income and outcome of the must be similar also.