For lightness I think you are going to have to go for titanium, CF (not so great on singles) or Ally, and one silencer is definitely the way forward, obviously.
I'm pretty sure the second silencer on the Leo's is there for visual balance; the link to it is as a strange angle not conducive to good gas flow, and the pipe is a narrower bore than the primary link pipe.
For reference, this straight through, same as the Leo's, Remus pipe weighs 1400grammes.



That's a lightweight pipe.