Bank of America and Citibank, the nation's two largest banks, raised their prime rates to 18 percent from 17 1/2 percent, making the higher rate the prevailing one for top business loans. Chase Manhattan raised its prime April 29 and dozens of major banks flooded in behind the third-largest bank. Among the remaining top 10 banks that followed Citibank Thursday were No. 5 Morgan Guaranty Trust and No. 6 Chemical.