So, if you have 2 lietenants in the same town, they can both add a siege counter and vape the place in half the time ?

Yes, but you still only make one seige roll.

or two with obsidian shackles

Yes.

RtL pg 10
(3.) Lieutenant Orders
The overlord may issue one order to each of his lieutenants per game week. Each order may be to either move or besiege a city.
...
Besiege a City: If the lieutenant begins his turn in a city, he can besiege it. If he does, the overlord places a siege token on that city’s space, as long as it currently has fewer siege tokens than the city’s Defense rating. (If it currently has siege tokens equal to the city’s Defense rating, the overlord does not add additional siege tokens.)

Note that raze rolls are only one per turn no matter how many Lts are present.
2. Resolve Sieges
First, remove all siege tokens from cities where at least one of the overlord’s lieutenants is not also present. Then, for each city that has a number of siege tokens on it equal to its Defense rating, roll a black power die. On a surge, the city is razed. On any other result, simply leave the siege tokens in place

Obsidian Shackles allows an extra seige roll though...