Yes, one of two twins can be a dwarf while the other is not.
Twins can be identical or fraternal. Fraternal twins develop from two
different eggs and are genetically different, while identical twins
develop from the same egg and are genetically the same. Dwarfism or
achondroplasia is a genetic condition, so only one of two fraternal
twins could have it, but not only one of two identical twins.
A page on achondroplasia on the Illustrated Overview website quotes a
"classical description" by Josef Warkany as saying "Concordance has
been observed in identical twins and discordance in nonidentical
twins." In other words, if one identical twin is a dwarf, the other
is too, but it can happen that only one of two fraternal twins is a
A document on Occupations and Avocations on LPA Online, the website of
the Little People of America, Inc., lists Mary Ann Meyer as "mom to
unique set of twins, (one LP and the other average)," where I assume
"LP" stands for "little person."
Wikipedia on twins
There was a 1953 paper titled "Chrondrodysplasia (achondroplasia) in
one of dizygotic twins." "Dizygotic" refers to fraternal twins.
I hope this is a satisfactory answer to your question. If you need
any further information about this, please ask for a clarification.