Sir Reg Empey, (Ulster Unionist) and Peter Robinson (Democratic Unionist Party) indicated that they had serious concerns over the issue of cross-border power generation.
The leaders have written to Dublin objecting to the Irish Republic's electricity provider ESB buying Northern Ireland Electricity (NIE).
The protest was over the £1bn deal in a letter to Taoiseach Brian Cowen.
Both claimed the business deal had major political implications, and said it amounted to the purchase of "a key component of our national infrastructure".
When the deal was signed, DUP Energy Minister Arlene Foster said she had received assurances from her counterpart in Dublin, Eamon Ryan.
Mr Ryan said that the status and identity of the Northern Ireland company would be protected.
The business deal was also subject to monopoly reviews and other tests in the Republic and the UK before the sale, which is due to be completed by the end of the year.
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Back in June this year, the Ulster Unionist MLA Tom Elliott criticised the Department of Enterprise, Trade and Investment (DETI) for "singing dumb" over the then only rumoured plans for the Electricity Supply Board (ESB) to acquire Northern Ireland Electricity (NIE).
"There are genuine concerns that a virtual monopoly ownership of the grid by the Irish State, in both Northern Ireland and the Republic, will have a detrimental impact on the price of electricity," he said, at the time.
"Facilitating state owned companies to buy up the grid is not what the single electricity market was supposed to be about; it also flies in the face of current European policy, which is urging member states to divest of owning energy transmission systems."
However, in July, NIE's owners have agreed to sell the network in the anticipated cross-border deal worth more than £1bn.
NIE owner, Viridian negotiated the deal with Irish energy company, ESB, for the purchase of the provider and the sale should be completed by the end of the year, subject to all conditions being met as the deal still has to be cleared by the relevant energy authorities.
As part of the acquisition, ESB will also acquire certain associated companies of NIE, including NIE Powerteam Limited and Powerteam Electrical Services (UK) Limited, which provide electrical construction and maintenance services.
In July, Peter Robinson's party colleague, Arlene Foster said: "I have received assurances from ESB on a range of issues relating to the transaction.
"These include ESB investment plans, the importance of securing existing jobs, and maintaining the identity of the electricity network business in Northern Ireland.
"I will continue to keep a close watch as the transaction proceeds towards completion, to ensure the interests of electricity consumers in Northern Ireland are being properly considered.''
See: NIE Sells Power Network
(BMcC/GK)
