The Syrian Observatory for Human Rights reported that the first bombing left 23 dead in the village of Hawar Kilis, near the border with Turkey.
Three rebel commanders were among nine killed when a joint rebel headquarters was later hit in the town of Marea.
Rebels in Syria have fought bloody battles against IS since early 2014.
They reject the group's tactics and its treatment of civilians living in areas under its control.
The Syrian Observatory said Wednesday's bombings appeared to be an attempt by IS to "expand its reach" in Aleppo province, where the group already has a significant presence.
Among those killed by the bombing in Marea were the local "emir" of the jihadist group al-Nusra Front, al-Qaeda's affiliate in Syria, and two commanders of the Islamist rebel groups Jabha al-Shamiya and Jaish al-Mujahideen.
The Syrian Observatory said clashes erupted between rebels and IS militants around the town after the blast, but that they subsided later on Wednesday.
Al-Nusra blamed IS for the attack and posted online a photo of what appeared to be the body of its emir, whom it said was known as Abu Marea, on a blanket.
A spokesman for the main Western-backed opposition alliance, the National Coalition, condemned the bombings.
"[IS] exploited the rebels' concentration on fighting pro-regime forces who have been trying desperately to encircle [the city of] Aleppo," Salem al-Meslet said.
At least 32 people were killed in two overnight car bomb attacks on rebel fighters in northern Syria by Islamic State (IS), a monitoring group says.