A pear and apple won't pollinate each other. So it would be better to have two of each. However some are adeqately self fertile. And often there are trees nearby that will provide pollination.

In my area most varieties will fruit just fine on their own. I'll bet if there are other pear or apple trees in your area a single tree of each will probably produce just fine. If you have room though, trees in general will always produce better if cross pollinated by a diffrent variety of the same fruit. For instance, if you planted a Summer Crisp pear you may want to pollinate a Harrow delight pear. I am strictly using those two varieties as examples because it seems I remember hearing they do well in WI. Definatley check with a local expert before you chose varieties.

Hope this helps,
Scaper