It is the first time that two £1 million prizes were up for grabs as part of changes that came about through the Budget reforms.
A man from Reading with total holdings of £8,175 has become a millionaire after buying the winning bond in April 2013.
The second winner from West Sussex has £30,000 of bonds and he bought his winning bond in June 2008.
Starting today, there were also 52,000 more tax-free prizes and the annual prize fund rate has increased from 1.30% to 1.35%.
Premium Bond holders have had the opportunity to increase the amount they invested from £30,000 to £40,000 since June 1 – another Budget measure.
The last time the maximum investment limit for Premium Bonds went up was in May 2003 when it increased from £20,000 to £30,000.
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Jane Platt, chief executive of NS&I, said: "Premium Bonds are one of the nation's favourite ways to save.
"This news is good news all round as we're able not just to add an additional jackpot prize of £1 million, but also to increase the overall number of prizes.
"Many of our savers have taken the opportunity to increase their savings in Premium Bonds and as a consequence increase their chances of winning."
The prize fund is estimated to increase from £51,988,425 in July 2014 to £55,517,300 in August 2014, and the number of prizes is estimated to increase from 1,845,745 to 1,898,030.
Over 21 million people hold Premium Bonds.
Across the UK there are over one million unclaimed Premium Bonds prizes worth more than £47 million.