(1) 二つ目のお話のthemeは愛と失意{しつい}です。
(2) 二つのお話のthemeは愛と失意です。
Fundamentally, both sentences mean
The two themes of this talk are love and despair.
What is the difference in meaning / nuance between #1 and #2?
The first one means
The themes of the second talk are love and despair.
The second one means
The themes of the two talks are love and despair → implying either 1) that one talk is about love, and the other is about despair, or 2) they are both about both love and despair.