Complex sentence: Only Prime Minsters Pierre Trudeau, Sir John A. Macdonald, John Thompson, and Paul Martin recommended with any frequency the appointment of Senators belonging to opposition parties; all together, only six opposition Senators have been appointed on the recommendation of other Prime Ministers.
Split into simpler sentences: Only Prime Minsters Pierre Trudeau, Sir John A. Macdonald, John Thompson, and Paul Martin, appointed opposition senators in any great numbers. Only six opposition Senators have been appointed on the recommendation of other Prime Ministers.