We present a polynomial-time algorithm for finding an optimal set of taxa that maximizes the weighted-sum of the phylogenetic diversity across two phylogenetic trees. This resolves one of the challenges proposed at the 'Current Challenges and Problems in Phylogenetics' workshop in the first week of the Phylogenetics programme. It also completely closes the gap between optimizing phylogenetic diversity on one tree, which is known to be in P, and optimizing phylogenetic diversity across three or more trees, which is known to be NP-hard.
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