You're reading it correctly, but I disagree with your conclusion. If Mayne says p=.7, and Szymborski says p=.8, and their estimates are independent - remember, my classifiers are not human experts, they are not correlated - then the final result must be greater than .8. You already thought p=.8 after hearing Szymborski. Mayne's additional opinion says Joe is more-likely than average to hit more than 10 home runs, and is based on completely different information than Szymborski's, so it should make Joe's chances increase, not decrease.