im no expert, but i don't see a problem with your math... the problem is due to the disconnect between principal and actual interest paid... you are being penalized for having a cheaper loan.
if it had been a more expensive loan (say 50.000 interest ) you would be able to go above the "caped" value from the 1st loan : (20000+50000)*.3333 = 23.000
if its a really cheap loan (say 5.000 interest ) you would be better off not claiming the deduction : (5000 + 20000)*.3333 = 8.000
using the principal to reduce the interest without considering the interest rate is where this goes "wrong"....
