Police revealed that Phornchanok Chaiyapa and taxi driver Somchai Kaewbangyang confessed on Thursday night after their daughter and Phornchanok's mother showed up at police request and persuaded them to tell the truth.
New information revealed that Somchai was previously married to Phornchanok.
The cabbie claimed Phorn-chanok had called him, complaining that Shimato was refusing to give her money. So, he said he armed himself with a knife and rode his motorbike to her home.
Finding them in bed, he suffocated the Japanese man with a pillow.
Somchai then dragged the victim's body into a bathroom, where he hacked the body into pieces, before putting these parts in fertiliser sacks and loading them on
to a pickup to drop the bags off at different locations.
"Jealousy and money issues are key motives," Provincial Police Region 1 acting chief Maj-General Amnuay Nimmano said.
He added that there was solid evidence and police should be able to conclude the investigation in 15 days.
Somchai and Phornchanok were taken to several spots to re-enact their alleged crime yesterday.
After Shimato's death made the headlines this week, the daughter of Phornchanok's previous husband, Katsutoshi Tanaka, came forward to call on police to re-open her father's case.
Tanaka reportedly died from falling down the stairs in 2003.
Keiko Matta, 31, said she suspected foul play, especially since Phornchanok reportedly claimed Bt3 million (S$118,000) from Tanaka's life insurance.
Matta is Tanaka's daughter from another woman.
Police spokesman Lt-General Prawut Thavornsiri said Tanaka's case would only be re-opened if there was new evidence.
"The statute of limitations for a murder is 20 years," he pointed out.
Nation Police Office's Level 10 adviser General Korkiat Wongworachat has been put in charge of cases related to Phornchanok, Prawut said.