Yes and No.
Yes is that you can use one letter for both purposes for sure.
No is that the letters must be carefully written to reflect both points of
NIW and EB1. For NIW, the point is that what you are doing and what you have
been doing are beneficial to the States; and for EB1a,the point is that you
are already the few top scientists in your area. They could be so different
in some cases.
Let's say you are saying you are the top scientist in finding a gene
responsible for a rare disease, but you could be challenged for the
substantial benefits for a NIW purpose. Or you are saying what you are doing
change the regular life of American people like designing a green coffee
cup for Starbucks, but that may not necessarily make you the top people in
that field.
