My best friends are identical twins and although their handwriting is very very similar, it’s not quite identical.
Sattua, that’s cool. What about your nose?
I’m actually an identical twin, and while my handwriting is similar to my brothers, it’s not the same.
Libertarian, it seems that inherent in your OP question is another question, “Do genetics determine handwriting characteristics?”
If so, it would seem that any study of twin handwriting would have to separate the twins into two groups: those that grew up together and those that were apart from birth.
Otherwise, the nature vs. nurture question cannot be answered. If twins gew up together, maybe they went to the same schools under the same teachers, and copied each other’s styles.
Also, any study of this sort should also look at non-twins as controls. It might be interesting to gather 1000 handwriting samples, of twins & non-twins, randomize them, and see if neutral observers could pick out the pairs of similar ones (blind, wihout knowing the names, of course). Then see if the similar pairs are twins of each other or not. This method would reduce unconscious bias from such subjective analysis.
I’ve had half of different sets of twins in my classroom for the last three years. This year, I have two identical twin boys in the same class (placed by ESL level; normally we wouldn’t do that).
Speaking anecdotally, not only are the handwriting samples not the same, neither are the test scores, learning ability, or learning attitudes. The twins I have this year are significantly different ability-wise. One is barely a reader–the other, though behind, is several grade levels ahead.
(Anyone see that Everybody Loves Raymond episode? “Is he stupid? You should tell us if he’s stupid…” Laughed my ass off.)
However, Ruffian, you didn’t say if these were identical or fraternal twins. If genetics is part of this discussion, fraternal twins aren’t much more than siblings with a similar birth date.
as an identical twin, my brother and i have very different handwriting which doesn’t even compare to my mother’s or father’s. we went to school togethor from k-8, split for high school and went to the same college, and are handwriting has been different almost the entire time.
Okay, not to resurrect this thread two weeks later…but I wanted to address your question, Musicat. (Hey, I’m a teacher and it’s September…been a busy month…)
All of the sets of twins I am speaking of are identical. Fraternal twins would be a nonissue to this thread or any similar discussion, for obvious reasons. That’s a pretty elementary concept that I’d hope all contributers to this thread would assume.
Most fascinating…the identical boys I have in my room this year are stark contrasts and would be a fascinating case study. One just qualified for RSP (pull-out special education program) and has…well, bizarre reasoning abilities. On the most recent science test, twin #1 got a C. Twin #2 got a 10%, thanks to answering questions like “What are the three branches of Earth science?” with “Red, yellow, green.” His comments in class are a bit…odd, enough that his twin just rolls his eyes as if to say, “Don’t look at me! I only look like the guy…”
We joke that twin #1 must have been sitting on twin #2’s head in utero.
Ruffian, I think you have the makings of a new sitcom pilot there…personally, I like Twin #2’s answer. I probably would have answered, “Poplar, Pine and Pumpernickel.” 
They’re twins! But they’re not alike! (theme music swells) Think of what Wayne Brady could do with this!
Friends of mine in high school were identical twins, very different handwriting. One was a rightie and the other a southpaw.
Handwriting has got to be somewhat genetic. When I met my father for the first time (well okay, I lived with him till I was 6 months old but hadn’t seen him since) last month, we found our handwriting to be very VERY similar. Not identical but pretty darn close. It was freaky!