I've, known of people to paper train their dogs in the house. The problem was the dog thought they were ONLY supposed to "go" on the  paper, in the house; therefore, they would not go when they were outside. The owners might "catch" the dog "having an accident" out side (at least the dog acts ashamed like they have just had an accident when in fact- going outside is what you want to see happen...) The suggestion I have heard for THAT problem is to put the paper outside and over the course of several days reduce the size of the newspaper until it looked something like a postage stamp...then the dog gradually gets used to going outside by going on the paper...where and I going with this? you dog is already going outside and unless you have taught her to wipe herself I'm sure there is no paper involved...My thought is that she has somehow learned that the yard spot is the only "okay" spot to go to the bathroom.  Theres a couple was Id suggest convincing her to go else where.

Examine her "spot." Whats different about it than other places? Is it grassy, sandy, woodchipped, etc. Bring her to places like that to relive herself. When you see her going to the bathroom outside of her comfort zone, be sure to lavish her with lots of praise and maybe a treat or two. Also, if you walk your dogs together reward and treat the other one for going potty. My other thought would be to look at it as you would housebreaking-do what you did to teach her (or another dog) to go outside... Generally you make a shedual up, and bring the dog out to do there business at certain intervals i(as opposed to letting them go anywhere and everywhere they want.) So put her on a leash, take her out to her "spot" and right as she goes to relieve herself- give the command "potty!"...eventually she will learn to associate both the leash and the command with going potty...then slowly give the command further and further away from her usual spot...eventually she will do it on walks...good luck. I hope that was coherent...