Whatever you do, make sure that your girls use nothing but College Board materials for the PSAT/SAT prep. Unfortunately, because of the format change, there is a dearth of official materials presently available.
Here is the Amazon link to the new “Blue Book,” which will release in June 2015:
It’s been some time since I worried about my D’s standardized testing but, at the beginning of her real prep (not the time she took the SAT for Duke TIP), I helped her with the “Xiggi method” (thank you, @Xiggi) in that she started by practicing one section at a time, made sure she understood all the questions and answers, all done without time pressure at the outset. It was time well spent because it helped D get used to types of questions and the wording of the questions, etc. IIRC, she was able to finish sections easily within the time limit after 4-5 sections because her familiarity with the question types worked in her favor and this greatly improved her accuracy.
Anyway, there is a link to the condensed version of the Xiggi method posted by mezzomom in Post #1 here:
http://talk.collegeconfidential.com/sat-preparation/68210-xiggis-sat-prep-advice-p1.html
It’s a very long thread, but the application of the approach can be understood within the first two pages or so.
Because you mentioned that your twins have not taken annual standardized tests, I think this approach might work well and reduce their stress.
Also, you’ll note that prepping for the 11th grade PSAT and SAT can be accomplished simultaneously during the upcoming summer.
Good luck to both of them!