Party leaders Alex Salmond and Ieuan Wyn Jones on a hung parliament's 'opportunities'
Welsh and Scottish nationalist parties say they will demand "fair funding" before negotiating with Labour or the Conservatives in a hung Parliament.
The SNP and Plaid Cymru are also asking for the protection of the "most vulnerable" local services.
SNP leader Alex Salmond said a hung Parliament, where no single party wins a Commons majority, would provide an "opportunity" for change.
Recent polls suggest this is a possible outcome at the general election.
If it happens, it could give the smaller parties a say in who forms the next government.
'Best possible outcome'
Currently the SNP runs a minority administration in the Scotland, while Plaid Cymru runs the Welsh Assembly Government in coalition with Labour.
At a joint press conference by the nationalist parties in London, they ruled out forming a formal coalition government at Westminster with Labour or the Conservatives.
They said they would instead focus on working towards "achievable objectives".
Plaid Cymru leader Ieuan Wyn Jones said: "In their race for the keys to Number 10, the London parties have forgotten about what really matters to voters in Scotland and Wales, but the SNP and Plaid Cymru have not.
"With the prospect of a balanced Parliament becoming more likely by the day, we believe that an election that gives no single party an overall majority would be the best possible outcome for our nations."
He added: "We would demand fairer funding for Wales and Scotland to protect jobs, our schools, our hospitals and the most vulnerable in society.
"At the heart of our platform there would also need to be a real commitment to grow our economies through fast transport links and additional support to create thousands of high-quality jobs in the green and creative industry sectors."
Mr Salmond, who is Scotland's First Minister, said: "With the real possibility of a balanced parliament at Westminster, the situation is redolent with real opportunity for Scotland and Wales.
"Both Labour and the Tories are threatening deep, harsh cuts which will endanger the Scottish and Welsh recoveries."
He added: "Working together, SNP and Plaid MPs will champion fairer funding and guarantee provision of public services which are vital to most vulnerable in our communities."
Mr Salmond said he and Mr Wyn Jones were "more experienced than anyone else in handling balanced parliaments".
Mr Wyn Jones said: "We would be failing in our duty if we didn't outline our objectives in advance [of the election]."
But Nia Griffith, Labour MP for Llanelli, said the Plaid-SNP agreement was "totally meaningless."
For the Conservatives, shadow Welsh secretary Cheryl Gillan MP said: "It is ironic that parties which cannot represent all parts of the United Kingdom are seeking to influence a future UK government after the election with such vague, contradictory wish list."
And Welsh Liberal Democrats leader Kirsty Williams said: "A handful of nationalists in a hung Parliament situation can't bring about change that will really work for Wales."
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