Converting Single Sink Plumbing to Double Sink

I've got a 92" bathroom vanity with a single sink in the center (46"). I am going to install a new vanity and counter top. While I'm at, I am considering converting the set up to two sinks.
A cabinet vendor told me that I don't have to tear out the sheet rock and plumb individual watse/vent pipes for each sink, but that I can use the existing waste and water supply lines, and just run branch out from these to each new sink.
My question is:
Should I install a separate p-trap for each sink, and run these to the exisiting waste line, or run downwarding sloping waste lines from each sink to join the existing p-trap? In either case, the venting would be handled by the existing vent.
For the supply lines, I suppose this is just a matter of branching off of the existing supply lines. Correct?
Thanks, Bruce
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