Rev Ian Paisley and Martin McGuinness have jointly requested Northern Secretary Peter Hain to move out of his offices at Stormont Castle in preparation for the return of devolved government next month.
The letter drafted by the DUP leader and Sinn Féin's chief negotiator is the first of its kind and also marks the first communication in their roles as first minister and deputy first minister.
Mr Hain responded quickly, saying he and his junior ministers would be "delighted" to do so and claiming that the letter was proof of the two parties' willingness to work together.
In a statement yesterday he said: "The fact that they are keen to occupy the seat of government at Stormont Castle as soon as possible is a very positive signal of their serious intent."
The parties are expected to meet tomorrow at Stormont for a run through of the d'Hondt process to see which departments ministers-designate will opt for.
The process will be indicative only and not legally binding as that can only happen when the Assembly meets next month.
"I am pleased that the party leaderships are getting down to the business of preparing for government," Mr Hain said. "And to that end I have instructed my officials to make themselves available to assist with advice on policy and analysis so that the executive is in the best position to operate effectively from day one, May 8th.
The DUP has indicated it will opt for a policy of rotating its ministers to ensure that a range of party figures will hold ministerial office.
Deputy leader Peter Robinson said at the weekend: "We do have a lot of very able people and the opportunity should be there for them all to give of their talents." Referring to the d'Hondt "dry run", Mr Robinson said if new ministers were to get the needed preparatory work done before May 8th then they would need to be properly briefed by their departments.
"Then you need an indicative running of d'Hondt. I think all of the parties are up for that."
Sinn Féin said last night it would do everything needed to ensure it was "solidly prepared" for government.
Deputy first minister-designate Martin McGuinness is expected to make clear the party's preparations at a Stormont news conference this morning.
Mr Hain said the Paisley-McGuinness letter was another of those "it would never happen moments" which came at the end of a "remarkable week for Northern Ireland".
"This is what the British and Irish governments have been working towards since the signing of the 1998 Good Friday agreement. But then Ian Paisley's DUP was outside the tent and Gerry Adams's Sinn Féin was half in."
He told the BBC yesterday: "In the last couple of years since I became Secretary of State for Northern Ireland there have been plenty of other 'it would never happen' moments. The IRA successfully declaring its war over, decommissioning its arsenal of weapons, driving criminality out of the organisation and finally republicans signing up to support the PSNI.
"Last year a peaceful marching season, not a soldier on the streets of Northern Ireland on July 12th for the first time anybody could remember. Ian Paisley meeting the Catholic Archbishop of Ireland for the first time in his 80 years of fiery Presbyterian preaching. The stage was set for the political endgame.
"The two most polarised parties have done what they have never been willing to do before, to share power on May 8th. I believe it will stick," he said.