Morgan Guaranty Trust & Co., the nation's No. 5 bank, cut its prime interest rate Tuesday by two full percentage points, to 14 percent. First National Bank of Chicago, the No. 9 bank, followed the move to 14 percent, cutting its rate from 15 1/2. The decline has been prompted by signs that a deeper-than-expected recession will ease the nation's inflation rate, a key variable in determining interest rates.