The IEEE 802.11be standard introduces a revolutionary multi-link technology that allows simultaneous operation on multiple channels. To ensure data delivery, a multi-link device uses a single finite sliding window that limits the number of sent packets. Therefore, the more packets are sent on one link, the fewer packets are left for the others, leading to a shortage of packets for transmission and a reduction in throughput. This letter is the first to address such an issue. First, it introduces a theory of the optimal aggregation algorithm. Second, based on the developed theory, this letter presents an aggregation algorithm that improves throughput by selecting packets to aggregate. NS-3 simulations confirm up to 50% throughput improvement over baseline algorithms.