Police chief Godwin Matanga declined to give details but said investigations were continuing.
“I would like to applaud the level of cooperation and support that the joint investigation team has so far received, culminating in two suspects being accounted for,” Matanga told police officers in Bulawayo.
Two other suspects were arrested last week but have since been released.
The blast occurred soon after President Mnangagwa had finished addressing the rally and was on his way out.
He escaped the blast but two of his lieutenants, Vice-President Kembo Mohadi and national chairman Oppah Muchinguri-Kashiri were injured in the blast and had to be taken to hospital in South Africa.
Mnangagwa said elections will go ahead as scheduled on 30 July.