Ian Ogilvie, 50, and Allan Donnelly, 47, preyed on the teenager for months - even filming their sick antics.
Ogilvie was already on the sex offenders' list for life at the time and was also the subject of a supposedly strict order banning him from going near under-16s.
The pair were snared after a man allegedly asked to take part in the rape of the child alerted police.
The shocking tale emerged as Ogilvie and Donnelly admitted to a string of sex charges when they appeared at the High Court in Glasgow.
A judge said both took part in a "deeply horrifying level of depravity" and each could face life sentences when they return to the dock later this year.
The young victim had moved to Scotland around six years ago from her home in Thailand.
She joined a woman, who had earlier met Donnelly while he was on holiday in the Far East.
Donnelly - who had a previous conviction for lewd and libidinous conduct from the early 1980s - developed a sick fascination with the schoolgirl.
He started having sex with the youngster when she was 15 at his home in Airdrie, Lanarkshire.
Last June, Donnelly then introduced the teenager to Ogilvie, who was already on the sex offenders' register for life.Ogilvie was jailed in Northampton in 2005 after being caught with child porn.
He and Donnelly soon abused the vulnerable youngster together often at Ogilvie's home in Motherwell, Lanarkshire.