Should there be a distance between patio fruit trees like apple, pear and plum?
Can Patio Fruit Trees Be Grown Near to One Another
Certified GKH Gardening Expert
You don't need to keep them separated. The only time there is a concern with cross pollination is if you plan on growing the seeds from the fruit.
For trees in containers, you want to have a reasonable distance of a few feet between, just so there is good airflow on the plants, but other than that, there is no reason or need to keep them apart.