Why does Kaspa need an extra Merkle Tree?

Kaspa adds an additional Merkle Tree to solve an ordering problem that Bitcoin never has to face. In Bitcoin, each block points to only one previous block, so order is always unambiguous and a single Merkle Tree works fine. Kaspa allows blocks to point to multiple previous blocks and lets parallel blocks coexist in the network — but a Merkle Tree is order-dependent, and order between parallel blocks in a DAG cannot be known. The solution is a second Merkle Tree that handles this additional ordering challenge. For a beginner, the takeaway is that Kaspa's blockDAG design requires more bookkeeping than Bitcoin's chain, and the extra Merkle Tree is one piece of the engineering that makes that design work.