well here's what i would do . . .
everytime they need to go outside to potty or poo take them on leash over to the prepared spot. no potty/poo no freedom in yard. if they do go where you want then have a party and hand out a 'shrimp from the barbie' heehee. once someone goes in the spot it will be easier to designate that area as the toilet. i think you can even buy stuff to put down to encourage them to go in a certain place. we put down mulch or sawdust and my two seem to love it. the love it even more when we put down fresh mulch they race to christen it!