Not sure if double wall makes sense to me.
The main advantage of Titanium over Polypropylene is that you can use them in emergency as a pot.
You cannot do that with a double wall mug.
On the downside ( of the single wall Ti)is that you could burn your lips if the drink is too hot .
Comparing weights a GSI insulated 500ml mug is about 1 oz lighter than a 450ml double wall Ti. Witch one will keep your drinks hot for longer I do not know but the GSI Infinity Insulated Mug works for me (making soup for example), is lighter and cheaper than the Ti equivalent.
Franco
Sea 2Summit have something similar out now, the Delta, but about the same weight as the Ti doble wall.
Afterthoughts...
If you are really fussy about taste, you can detect some "plastic" taste with Polypropylene however some have reported "bad taste" from Titanium as well. I have not noticed that from my 550ml Ti mug but I am not that fussy.
( "pure" Titanium is inert, however none of the Ti cookware is made from pure Ti)