In your case , I would suggest going in for a single 1750 kVA generator.
The reason for this is that , if you go in for a 500 kVA generator , the difference in rating between this and the existing generator ( 1250 kVA ) is quite high , and tuning the system of 2 generators so that you get stable operation at all loads is difficult.
When you synchronize two or more generators , they should be synchronized at all times i.e. when new loads are added , when existing loads are removed. Each time such a change takes place , the load sharing between the two generators has to be maintained.
It all depends on your load system i.e. is it a steady load , or is it a mixture of several load which come on at different times , go off at different times ...
A synchronizing panel is an expensive matter and difficult to maintain. In my opinion , it would be better to go in for a new 1750 kVA generator. You could always sell off the existing 1250 kVA generator.