This is possible for Freight Invoices as both lines will generate a brokerage commission each in the Description section of the Freight Invoice.
However, for Demurrage Invoice adding the same broker twice in the Laytime Calculation (Claim Tab):
Broker A 3.75%
Broker A 1.25%
is not supported by the system. As such, please consider the below two options:
Aggregating the two into one single entry.
If the above is not possible or not accepted by the Broker, it would be worth considering creating a "dummy“ broker against which you can set the additional percentage in the Laytime Calculation screen (Claim Tab) which in turn will appear in the Demurrage Invoice.