The two are on trial in the Protea Magistrate's Court on four counts of murder, two of attempted murder, reckless or negligent driving and driving under the influence of drugs.
They were allegedly drag racing in Protea, Soweto, in Mini Coopers while drunk and high when their cars collided, killing four schoolchildren and critically injuring two others. They face an additional charge of failing to check the extent of the injuries of the victims.
Yesterday Tshabalala's attorney, Mlungiseleli Soviti, argued that the integrity of urine samples that showed the two had traces alcohol, cocaine and morphine in their systems were compromised because the accountability chain of how they were handled, stored and recorded had been broken.
The samples were taken from the men at Chris Hani Baragwanath Hospital on the evening of the crash.
Soviti said the expert testified that the samples were not properly sealed and were therefore not tamper proof.
"This clearly means that whoever wants to play around or contaminate them was at free will. The manner in which these exhibits were handled leaves much to be desired. The court should simply reject the evidence."
Soviti also said there was no evidence to prove the two accused were drag racing. He said the only suggestion was testimony from two girls who said they saw Maarohanye waving to Tshabalala as he drove past him.
Sovithi also argued that the court had failed to prove that his client was guilty beyond reasonable doubt.
Sovithi said his client had shown remorse for the incident and had wanted to apologise to the families involved. Tshabalala had not done so as he feared the act would affect the trial.