Combine two sets of clusters
To compare two LDA topics, you're really trying to compute the distance between two probability distributions. 

One such measure that's commonly used in these circumstances is the Hellinger Distance. To find the closest match for $x_1$ in the topics for $y$, you would calulate the Hellinger Distance between $x_1$ and each $y$ topic, then take the lowest one.

Keep in mind that there's no guarantee whatsoever that the "most similar" topic in this sense would be remotely, subjectively similar.