Lou:

Italian prune, a European plum, and Santa Rosa, a Japanese type plum, are not pollen compatible. Even if they were, Japanese plums tend to bloom earlier than the European types, so there would be no possibility of cross-pollination.

The good news is that most European plums are at least partially self-fertile, and among the Japanese plums, Santa Rosa has more reports of self-pollination than I have seen for other varieties. But, of course, both would benefit from companion pollinators of similar type, and would set more fruit. How much more, exactly, I can't tell you.

Don Yellman, Great Falls, VA