Which strand of a gene specifies a protein?
If a piece of DNA were to be transcribed on both strands, at the same time, the transcription complexes would collide and inhibit each other, like steric hindrance.

If you could overcome this interference and synthesize primary transcripts from both strands, it follows that the two RNAs would hybridize to each other and form complementary RNA-RNA hybrids.  These dsRNAs are very poor substrates for translation by the ribosomes. This was the initial mechanism proposed for anti-sense RNA to down regulate gene expression. 

In many, if not most, EUKARYOTIC cells, a dsRNA will trigger the siRNA/shRNA/miRNA response mediated by Dicer et al. leading to degradation of both RNA strands.

So in virtually all instances, only one strand will be transcribed, and encode a mRNA with a significant ORF.