Fortunately you are starting with enough time for your twins to prepare as well as possible for their PSAT/NMSQT in Oct 2015 when they will be juniors. You should visit the Financial Aid & Scholarships forum on CC and learn as much as you can about the many opportunities open to students who score highly enough to qualify for NMSF in their state and take steps to help them start preparing. An optimal time for PSAT prep is the upcoming summer and, because the upcoming PSAT will be under the new regime, you should definitely get a head start.
http://talk.collegeconfidential.com/national-merit-scholarships/
https://www.collegeboard.org/delivering-opportunity/redesigned-psat-nmsqt
That said, even if your girls don’t quite make NMSF in your state, the time they spend prepping for the PSAT will pay off when they have to eventually take the SAT, which should help with other merit scholarships.