Quick exhaust question

I just had a quick question about exhaust on the WS6/T-A/Firehawk LS1's... Do they come with true duals or a Y pipe??

y pipe

Any reason why GM didn't put in dual exhaust?? and just the Y pipe?

cost most likely, and easy instalation. Plus try to package two mufflers and still meet all legalites with out major sacrafices somewhere.