Complex sentence: The two co-founders of Lincoln Partners had worked together from 1981 through 1990 in the Mergers and Acquisitions Department of Paine Webber.
Split into simpler sentences: The two co-founders of Lincoln Partners, had worked together since 1981. From 1981 through 1990, the two co-founders had worked in the Mergers and Acquisitions Department of Paine Webber.