For money exchange, I have two fields "DOLLAR" and "EURO". Is there a way that entering the amount in one of the fields would give the result in the other, and vice versa. Or do we need to have two separate procedures.

It's possible, using either auto-enters or script triggers on both fields. However, it's not the proper approach to adopt, primarily because it leaves no trail. Normally, you would have fields for Amount, Currency, Rate and a calculation field = Amount * Rate for use in your internal bookkeeping and tax reports.