Connection of two identical transformers
There is no physical reason you can't back-energise your 110 kV (HV) system from 25 kV (LV). However, watch out for the following issues:


Protection settings do not usually expect reverse power flow, or back-energisation. You may find that the LV protection trips on reverse power or directional overcurrent, or the LV overcurrent protection trips on magnetising inrush current.
Any power transmitted from 120 kV -> 25 kV -> 120kV will suffer two lots of voltage drop. You can compensate for this using the transformer's tap-changers.


If you do not understand the above, this indicates that you should not be working on HV equipment, unless closely supervised by someone who does understand the above.

More so than usual for my other posts, this is not professional engineering advice. In case of doubt, consult a professional engineer who is familiar with your installation to provide detailed advice.

Do not mess with HV equipment unless you have been trained, deemed competent, and authorised by the owner/operator of the equipment. Do not perform HV switching unless your switching sheet has been checked and authorised by a person who is familiar with the equipment, and trained, deemed competent, and authorised by the person taking charge of electrical works.

You will kill someone if you do not know what you are doing.