The two junior ministers in OFMDFM, the DUP's Ian Paisley Jnr and Sinn Fein's Gerry Kelly, have each been given permission to appoint an adviser, after a special order was passed by the Privy Council.
However, Ulster Unionist deputy leader Danny Kennedy accused the two main parties of government of an "underhand carve-up".
He said that the Assembly was neither asked nor notified about the appointments and the DUP and Sinn Fein had gone behind MLAs' backs to seek approval for the move.
He queried the need for further assistants in the department – where First Minister Ian Paisley and Deputy First Minister Martin McGuinness already have three advisers apiece – a week after he raised the fact that OFMDFM has 415 staff, which is twice as many as 10 Downing Street.
Mr Kennedy noted that during the previous administration, led by the UUP and SDLP, "Mr Paisley Jnr said there was not even a need for junior ministers, never mind advisers."
But the DUP man responded: "First of all there was nothing underhand about this. The DUP did not go to the Privy Council, it was the Secretary of State who asked the Privy Council to agree to two advisers."
The junior ministers' role had been expanded and now included responsibility for liaising between the Assembly and the Executive.
The needs of children, young people and older people were also part of their remit.
"At the first meeting of the Executive in May it was unanimously agreed that the role of junior ministers would expand and Danny Kennedy's own party leader (Sir Reg Empey) voted for that change," said Mr Paisley.
"This has meant an enhanced workload and also an important role which I believe has been demonstrated as very worthwhile in the first 120 days of the Executive and the Parliament (Assembly) and has been responsible for the largely smooth running of the administration."
He said that previously there had been problems between Executive and Assembly and a breakdown in communication, slowing the wheels of government.
An OFMDFM source added: "It is worth noting too that when Danny Kennedy talks of an underhand carve-up, it was the DUP which ensured there was a statutory committee with powers to call ministers from OFMDFM to account and scrutinise their work.
"Danny Kennedy is chairman of that committee."
They added: "Danny Kennedy has the power to scrutinise it in the committee, thanks to the DUP pushing for that provision."
The source also argued that the last OFMDFM, under then UUP leader David Trimble, had more staff than it does now.
Mr Kennedy said: "There has been a lack of transparency in these appointments.
"And I also think they beg the question from the long-suffering taxpayer: 'How much do these advisers cost?'.
"And how can a department twice the size of Downing Street ... justify the expenditure in a country the size of Northern Ireland?" he asked.