Transformer not allow operate more than 50% load

It is obvious that someone has decided to operate at N-1 so that if you lose 1 transformer the other take the full load. This is acceptable but in normal power systems you would tend to run the transformers at about 70% and if you need to cover all the load with one you can generally overload the transformer to 120% continuously. You will accelerate the aging & you will need to look at what you need to drop of or shed in the load whilst the other Tx is repaired or replaced.

Having said that the bus-tie, busbar rating & protection system may constrain the load that you can operate at. You need to check this as I have seen some places where they put in larger Tx that the bus current rating & set limits. Not good practice but it does happen.

You need to do some research into the history of the design to find out why.