I am remodeling my master bathroom and want to add sink. Currently, there is one set of pipes for a sink and would like to splice them into a second sink. The problem is that the current plumbing is located in an exterior wall and I need to know how to cut open the wall, so that I can expose the plumbing. Then, I can add my additional plumbing for the new sink. Please Help - anyone!

Thanks,
Kristin

Is there a reason why you can't get at it by demoing out an interior wall?

Cutting open and repairing an exterior wall is going to require repairing the vapor barrier, the underlayment and the siding itself.