I would argue that winning a Masters Series event is as hard as winning a Grand Slam in terms of having to beat more quality players. In a Grand Slam, the first few rounds can throw up some lucky losers, qualifiers, wild cards and lower ranked players before being really tested.
So not only winning a Masters is tough but winning the double back to back is a massive achievement. Here are some of the players that have achieved this:
1. Jim Courier 1991
Jim had a banner year in 1991, not only winning the IW/M double (and the doubles event!) but taking home the French Open, US Open Runner Up and rising to number 2 in the world. The only player I think will emulate this is Nadal.
2. Michael Chang 1992
This same year reached the US Open SF, losing to Edberg in a 5-hour, 26-minute marathon (longest in Open history). Chang on US hard courts was in his element.
3. Pete Sampras 1994
Also this year won career-high 10 titles in 11 finals, including first Australian Open (d. T. Martin), second Wimbledon (d. Ivanisevic) and second ATP Tour World Championship in Frankfurt (d. Becker). Nice.
4. Marcelo Rios 1998
Compiled his best career season in 1998 by leading ATP with seven titles in eight finals and co-leader (w/Agassi) with 68 matches won. One of the most outstanding years in tennis history.
5. Andre Agassi 2001
Won first title in Indian Wells in his 15th attempt(!), defeating Hewitt in SF and rival Sampras in final. Continued his success in Miami where he dropped only one set!
6. Roger Federer 2005, 2006
In 2005 finished No. 1 for second straight year highlighted by an ATP-best 11 titles (along with Nadal), including two Grand Slam and a record four AMS crowns. In 2006 his 12 titles were most in a season since Thomas Muster in ‘95…Became first player in Open Era (since 1968) to win at least 10 titles three straight years…Also first player to reach all four Grand Slam finals in a year since Rod Laver in ‘69. Show off.
7. Novak Djokovic 2011
No sooner did I write this post and Nole goes and wins the double and makes me have to re-edit this. An incredible effort from him, beating Rafa in the final.