One night, Lando scored with two chicks, and Dash begged, I mean BEGGED, for Lando to let him have one. They eventually compromised and Lando agreed to let Dash sit in the room while he did them both. It was the last time that ever happened because, though Dash would deny it 'til his dying day, Lando could have sworn he felt Dash's hand on his butt for a few seconds.
When Han Solo was captured by Boba Fett, Leia Organa didn't completely trust Lando just yet, and demanded he contact one of his scoundrel friends to assist in freeing Han. She realized Solo would probably die, and she hadn't quite finished her slumming days since Han had left her high and dry. Lando wanted to tap that himself, so he knew the best person to call was Dash. It was a bit of a roll of the dice, since he could easily be a walking cockblock, but Lando figured he could make this work.
In the end, Dash didn't help rescue Solo OR help Lando get any. In fact, Leia was closer to nailing her twin brother than either of them.