Swaziland human rights lobbyists say they have already put in place "underground" plans to launch a .
The men, from Aliwal North, pleaded guilty to the murder but claimed they had not not intended to kill the woman, only to assault her.
Bulelani Latha, 29, and Ayanda Gqongwe, 23, were convicted of the murder and contravention of the Witchcraft Suppression Act earlier this year.
They were sentenced on Wednesday.
The two suspected their grandmother of practising witchcraft last year, soon after the death of Gqongwe's mother - also Latha's aunt.
Shortly before her death, she had reportedly told Latha she had been bewitched by the elderly woman.
On the day of his aunt's funeral, Latha told the court, he visited his grandmother's home. When he entered her yard, his leg started to ache.
The following day his leg was still swollen.
Latha believed his grandmother had a motive for bewitching him because he had previously accused her of witchcraft.
A sangoma, whom he approached for advice, reportedly confirmed that he had been bewitched.
This was when he decided to attack his grandmother, the court heard.
Latha said he met his cousin, Gqongwe, and the two discussed his suspicions over a five-litre bottle of wine and a bottle of brandy.
They parted company and Latha went to the victim's home.
He approached his grandmother and insisted that she tell him why she had bewitched him and his family.
He slapped and kicked her before beating her with a metal dish, which weighed about 2kg, and broke a plastic milk crate over her head.
Gqongwe arrived at the scene a while later and joined in the assault.
The postmortem report confirmed the women suffered lacerations to the head, a fractured skull, fractured ribs, bleeding on the brain and a laceration to the liver. She died in hospital.
Acting Judge Leon Kemp said Latha was the instigator who planned the attack.
In sentencing, Kemp said it was important to bear in mind the brutal way in which the murder was conducted.
He sentenced Latha to 20 years imprisonment for the murder, five of which would be suspended, and 18 months imprisonment for contravening the Witchcraft Suppression Act.
The sentences will run concurrently, meaning Latha will spend an effective 15 years behind bars.
Gqongwe was sentenced to 15 years imprisonment for the murder, five of which were suspended, and 12 months for contravening the witchcraft act. His sentences will also run concurrently.
He will effectively spend 10 years in prison.