because it says n <0 you know it is a question revolving around positive/negative. so the question is is n negative?
1 - we dont' know what m is so can't be sufficient (BCE)
2 - still don't know what m is so (CE)
together you can try plugging in numbers using positive negative as a guide.
if m is 2 and n is 3 then 2 < 3 AND -3 < 2
Since we know we can make n positive, we have to try to make n negative:
if n is -10 and m is -11 then the first equation works but the second does not (10 is not < -11)
we can't try with a positive m becuase if n is negative then m must be negative and you notice that if m must be negative when n is negative then it will be impossible for the second statement to work - a positive will never be less than a negative therefore together they give you a consistent answer to the question and the answer is C.
Best of Luck
Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA