Plum tree pollination

If one variety pollinates the other does this mean that both varieties would produce?
Or the second must also pollinate the first?

So the black diamond variety is being pollinated by friar.
But friar is being pollinated by  Calita, Ozark premier, Laroda, Santa rosa.
If i plant friar and black diamond would the black diamond variety only produce?

From what I've read, the Friar plum is self-pollinating, but all self-pollinating trees (plum, apple, pear etc.) do better with other varieties nearby to aid in fruit set. Plums are pollinated by insects, not the wind. There are limits tho, as Asian plum cultivars tend not pollinate European cultivars and vice-versa. Your Black Diamond is Prunus Domestica or a European plum cultivar and your Friar is Prunus Salicina or Asian. Your other varieties may pollinate them and you may have no issues. Professor Alderman of the University of Minnesota wrote in 1934 "very few hybrid plums would accept pollen freely from other hybrids, but they all accept pollen from native plums"*. I hope this is useful for you. Cheers!

* The Holistic Orchard by Michael Phillips, p. 280

James Freyr wrote:From what I've read, the Friar plum is self-pollinating, but all self-pollinating trees (plum, apple, pear etc.) do better with other varieties nearby to aid in fruit set. Plums are pollinated by insects, not the wind. There are limits tho, as Asian plum cultivars tend not pollinate European cultivars and vice-versa. Your Black Diamond is Prunus Domestica or a European plum cultivar and your Friar is Prunus Salicina or Asian. Your other varieties may pollinate them and you may have no issues. Professor Alderman of the University of Minnesota wrote in 1934 "very few hybrid plums would accept pollen freely from other hybrids, but they all accept pollen from native plums"*. I hope this is useful for you. Cheers!

* The Holistic Orchard by Michael Phillips, p. 280

Thank you very much for your answer. Yeah there are other plums nearby some are native,so you say that the friar will be aided by the native trees if i understand correctly.

yes, the natives and hybrids will all aide each other in pollinating. You should be fine in the area of necessary pollinizers and have a nice crop of plums, given all the other factors during a growing season are suitable too (age of trees, pollinating bugs, the weather, no plagues, etc.)