Number of Trees
If you plant a lone sour cherry tree, expect fruits to develop after bees pollinate the blossoms. However, sweet cherry trees must not be planted by themselves, as another compatible sweet cherry tree must bloom at the same time to ensure cross-pollination. You cannot plant two of the same sweet cherry tree cultivar and expect good cross-pollination. Another cultivar is needed.
Sour Cherry Types
Because sour cherry trees are self-fertile, you may grow any number and array of cultivars in a garden. Sour cherry trees make poor pollinators for nearby sweet cherry trees. About eight cultivars of sour cherries grow well across a wide geographical range across the United States, in U.S. Department of Agriculture plant hardiness zones 3b through 8. Montmorency, North Star, Balaton, English Morello, Early Richmond, Surefire, Kansas Sweet/Hansen and Meteor are names of cultivars.
Sweet Cherry Types
In comparison to sour cherries, many more cultivars of sweet cherries exist. Grow them in USDA zones 5 through 8. The issue of cross-pollination among sweet cherries is confounded since some cultivars aren't compatible with each other, while some are universally good for cross-pollination. Cultivars Index, Lapins, Skeena, Sweetheart, WhiteGold, Sonata, Stella, Symphony, Sunburst and BlackGold make reliably good pollinators for all other sweet cherry tree cultivars. Match universal trees' blooming time with that of the other sweet cherry variety -- you want trees to bloom simultaneously so bees transfer pollen among both trees' blossoms. Names of other sweet cherry cultivars that need a companion pollinating tree are Bing, Rainer, Lambert, and Napolean/Royal Ann, Black Tartarian, Van, Angela, Sam and Sweet Ann, among numerous others.
Some sweet cherries never effectively cross-pollinate. Purdue University mentions three groups that will not yield fruits if planted exclusively in the same orchard. Do not plant Bing, Lambert, Napoleon, Star and Emperor Francis together. Windsor, Van and Venus also will not cross-pollinate each other. An isolated group of Viva, Hedelfingen and Vista also won't result in cross-pollination. Contact your local cooperative extension office for recommendations of pollinators and cultivars well-suited to your climate and soils. A rare few sweet cherries do tend to be self-fruitful: Stella, Lapins, Craig's Crimson, Sweetheart, Vandalay and WhiteGold. Planting them in multiples does help increase fruit set.