Why does pETDuet-1 only have one T7 terminator but two T7 promoters?
I think the presence of a single terminator is related to the high processivity of T7 RNA polymerase.

T7 RNA polymerase will transcribe a circular plasmid repeatedly before dissociating. The T7 terminator after ORF2 ensures that the transcript terminates efficiently.

Based on your schematic, these are the potential mRNA transcripts from that construct:

Transcript from T7 promoter #1

 5'---RBS---ORF1---STOP----RBS---ORF2---STOP---[T7 terminator]---3'


Transcript from T7 promoter #2

 5'---RBS---ORF2---STOP---[T7 terminator]---3'


It is doubtful that this vector will produce precisely equimolar concentrations of each protein product, however, the topology ensures that there is at least some of each protein produced. Of course, this could be tested empirically.

I hope that helps!