Skylark DuQuesne

Skylark Series, Book 4 of 4

Dick Seaton & Marc DuQuensne are the deadliest enemies in the Universe--their feud has blazed among the stars & changed the history of a thousand planets. But now a threat from outside the Galaxy drives them into a dangerous alliance as hordes of strange races drive to a collision with mankind. Seaton & DuQuensne flight & slave side by side to fend off the invasion--as Seaton keeps constant, perilous watch for DuQuesne's inevitable double-cross.

Synopsis from Open Library.