Time depends on a lot of variables, such as ease of access to where the cabling has to run.
In very tight wall cavities, it can often be more difficult to pull two cables through, compared to one.
Sometimes you can pull a second run through with the original cable.... impossible to answer accurately without knowing the difficulty level of the original installation.
In either case, the LNB has to be changed over to a dual output and the wall outlet changes to a dual outlet, so you need to allow plenty of time.... for the unexpected things that can happen with any cabling installation.
Bookmarks