Hey All,
I see you complaining about the
MGMAT guidelines. How dare you! : )
But seriously, this is NOT a definite idiom. You do NOT have to say "but also" when you say "not only". If you had it correct, it would have been preferable to this, but it's not a hard and fast rule. In fact, GMATPrep (the test that GMAT gives out) has featured a question that uses this variation on the idiom, which is how we KNOW it is not hard and fast:
By pressing a tiny amount of nitrogen between two diamonds to a pressure of 25 milliion pounds per square inch, scientists not only were able to transform the gas into a solid, but they also created a semiconductor similar to silicon
A. not only were able to transform the gas into a solid, but they also created
B. not only were able to transform the gas into a solid but also creating
C. were able not only to transform the gas into a solid but to create
D. were able not only to transform the gas into a solid but also creating
E. were able not only to transform the gas into a solid but they were also able to create
The correct answer is C, which uses "not only...but". This is another legit way to phrase it as is:
Not only X, but Y, too.
Not only do I want X, but so does Y.
Hope that helps!
-t