I have a 2000 grand marquis ls (Panthar platform) How can i convert 1 exhaust to dual?

I did this same thing to a 98 Grand Marquis, Ordered (online) H-pipe and two Walker Quiet Flow mufflers ( tons of research on the mufflers finally showing a 30% increased flow over stock) Had my local shop install the H-pipe, mufflers and fabricate the new tail pipe. I could only hear the mufflers at low rpm. Very low noise and no droning. I also installed a K&N air filter (using stock air box) . MPG increased 3 mpg average .. 8/10 second quicker in the quarter mile. All this parts and labor was less than $300,00 but was several years ago. Good Luck