dogs refusing to go outside

Hi, first im sooo happy to have found this forum. lots of good information here.

anyways I have 2 puppies, they are about 6 months old or so. They are brother and sister coonhound/austrailian sheperd mix. I am having a heck of a time getting them to go to the bathroom outside. The girl pees outside but does it again inside. the boy refuses to do anything outside. I will actually walk them outside for a good 20-30 minutes and they will wait to come back inside to pee and poop. i have tried spanking them. i have tried putting their face in it to smell then back outside. i have tried taking them outside then face in it then outside over and over about 4 times. I cannot think of anything else. My wife is about tired of it and as much as we love them we cannot stand it anymore and if we cant fix this they will have to go.

PLEASE help.