The Swede netted his 14th league goal of the season with a penalty seven minutes after the break and his Uruguayan strike partner sealed the victory in added time after Thiago Motta had put PSG ahead with a superb first-half strike.
The capital club have 43 points from 18 games ahead of second-placed Monaco who won 2-0 at 10-man Guingamp.
With Ibrahimovic among other key PSG players returning to the lineup after missing the 2-1 Champions League defeat at Benfica on Tuesday, the league leaders were expected to breeze past 14th-placed Rennes and they prevailed easily.
"The lads' commitment has been very good, with or without the ball. We've not conceded that many chances and we controlled the ball well most of the time," coach Laurent Blanc told reporters.
"When we put all the good ingredients (together) like we did tonight, it's tough to beat us."
PSG scored in the 19th minute with their first effort on target as Motta netted with a low shot from 20 metres.
They continued to dominate possession and were rewarded shortly after the interval when Blaise Matuidi was fouled in the box following a fine move, allowing Ibrahimovic to make it 2-0 from the penalty spot.
Romain Alessandrini got the score back to 2-1 with a long-range free-kick midway through the second half but the hosts failed to threaten keeper Salvatore Sirigu again and PSG secured the points with Cavani's 12th of the season in added time.
Ibrahimovic and Cavani have now scored more goals between them than 18 of the 20 Ligue 1 teams, with Monaco the only side to have collected more than their combined 26.
The principality club were without injured striker Radamel Falcao for the fourth match in a row but fellow Colombian James Rodriguez once again shone by setting up both goals.
The 22-year-old forward found Anthony Martial in the box for the opener after four minutes and left back Layvin Kurzawa headed home his corner in the 42nd. Rodriguez has delivered seven assists, more than any player in Ligue 1 this term.