Depending on the telco, and depending on the length of time you needed
> =>'overlapping' a charge for it.
They do charge you for regular service to the second location; after all, they are providing you with two separate service lines. But there isn't a special charge for the overlap itself.
This is limited to approx one month.
I knew someone who had two houses in the same exchange territory and he wanted this arrangement on a permanent basis. The phoneco wouldn't do it. I was told that this arrangement is not very efficient and problem-prone. Businesses can get it but for a fee and more work is required.