Niamh Lafferty, 15, and Georgia Rowe, 14, died after falling more than 100ft from the Erskine Bridge on October 4, 2009.
They were both residents at the Good Shepherd Centre in Bishopton, Renfrewshire.
Niamh was moved from the centre's open unit into its secure unit in March 2009.
This happened after her boyfriend died and after Niamh was found to be self-harming.
She had already absconded from the centre and was allegedly taking drugs, while three suicide notes written by Niamh to her mother, father and friend were discovered.
Adah Lambie, a senior social worker for Argyll and Bute Council, told the hearing at Paisley Sheriff Court yesterday that Niamh had "responded well to such a rigid environment".
But the inquiry was told that in July 2009, Ms Lambie had supported other social workers who wanted to move the teenager from the secure unit into a closed support unit, despite agreeing with reports which were read out in court which said Niamh, "wasn't ready to make positive decisions about her own safety".
Simon Gilbride, representing Niamh's family, asked Ms Lambie to explain the difference between the centre's secure unit and the closed support unit, where the teenager was being moved to.
She said: "It would be a continuation of working with staff she already knew."
But when pushed to give details about the structural differences in the units, and in particular the levels of security, Ms Lambie replied: "I'm not sure. I'm sorry."
She added: "It was not a secure unit as such but had very close links with the secure unit. It would not provide a locked environment as such but it would have a higher staff ratio than in an open unit."
However, despite the recommendation by social workers, Niamh was moved to a care home in East King Street in Helensburgh, her home town, while she waited for a place in the closed support unit at the Good Shepherd.
The inquiry was told that 24 days later, July 27, Niamh was reported to have taken an overdose and had apparently been making threats that she "wanted to be with Jonny", her dead boyfriend.
When questioned further by Mr Gilbride, Ms Lambie said she did not remember any alleged conversation with Niamh's mother Collette Bysouth, apparently telling her the girl could be moved to a respite centre in Ayrshire.
Mr Gilbride said: "Ms Bysouth has already given evidence in which she said she told you she was due to go on holiday the next day but was on the verge of cancelling it.
"But in her evidence she stated that you put your arms on her shoulders and told her about a place in Ayrshire where Niamh would be safe, where she would be taken care of."
Ms Lambie replied: "I don't remember having that conversation."
The fatal accident inquiry continues.