Banks and other financial institutions make short and long term loans to each other all the time at very low interest rates reserved for such institutions. The amounts can be very large and the duration can be just a day.
They should pay a tax, perhaps 0.01% (i.e. 1/10000th) of the principal each time funds are transferred between lender and borrower. The lender should pay half and the borrow should pay half for the privilege of using the financial system in this way.
The taxes collected should be used to enforce existing financial regulations.