Can cfDNA testing distinguish results in the case of twins?
I think you are overestimating how these cfDNA tests work.  Maybe I read the paper wrong, but from what I read of one technique (Sequenom's MaterniT21 test) there is no purification of fetal DNA.  They do restriction digests to assess the percentage of fetal DNA,  but I don't think they enrich for the fetal DNA when they sequence.  The library they make is mostly maternal, they are just looking for slight deviations in how many reads come from each chromosome, if too many come from chromosome 21, they conclude it's because the fetus has too many copies.

And as for SNPs, coverage is way to low for that.  We're talking about 40 million reads, even if that were mostly fetal DNA, which I don't think it is, that's not enough coverage to call SNPs.

So if there were two fetuses, and they were not identical, then the signal from each would be half what would be expected based on the restriction digests assay. So maybe you would have enough counts that you could tell that the Y chromosome counts were too low, and the X chromosome a bit too high, but since these tests almost certainly have not been clinically tested on multiple fetuses well enough for verify their accuracy, no company is going to report results.