In separate speeches in Glasgow and Edinburgh yesterday, the former Labour Prime Minister and leader of the Liberal Democrats launched separate initiatives designed to build support for greater devolution.
Mr Brown said that the Scottish Parliament should raise 40% of the money it spends, a move he insisted would give it "maximum accountability" while remaining in the UK.
Sir Menzies called for the parties to agree general principles for further devolution. He suggested broad heads of agreement should be drawn up between the parties before the referendum.
A more detailed policy programme, he said, should be agreed at a summit convened within 30 days of the poll if Scots vote No and be implemented by the UK Government after the 2015 General Election.
The interventions added to growing momentum behind moves towards greater devolution.
Despite that, however, the two parties remained at odds over the details of extending the powers of Holyrood.
Scottish Labour refused to embrace Sir Menzies's summit plan, as a spokesman said the party was focused on delivering its own vision for a "new devolution settlement".
The LibDem's tax proposals - which would put Holyrood in control of up to 60% of the money it spends - go further than Mr Brown suggested.
Deputy First Minister Nicola Sturgeon insisted there was no chance the pro-UK parties would reach agreement on more powers and repeated her claims that only a Yes vote would guarantee more powers for the Scottish Parliament.
Mr Brown set out his vision in a speech to community leaders, trade unionists and Labour supporters in a church hall in Tollcross, in the east end of Glasgow.
In a fiery, unscripted address - which earned a standing ovation - the MP called for major reforms to transform the UK's "unitary and centralised" system into a "partnership of equals".
He defended the UK's role of pooling and sharing resources to ensure what he called "covenanted" rights across the whole country, including common pension and welfare entitlements.
To applause, he said: "We want to separate Scotland not from the rest of the UK but from poverty. We want people to have the freedom from deprivation and homelessness, not from our neighbours."
He published a detailed submission to Scottish Labour's devolution commission, the group of senior MPs, MSPs and party members drawing up proposals for further devolution.
Among a six-point plan, he called for new legislation to make the Scottish Parliament permanent and recommended the creation of "power-sharing partnerships" to encourage Holyrood and Westminster to work together.
The former Prime Minister's proposals are expected to influence Labour's panel as it wrestles with a programme of tax powers for Holyrood which would command support from all sections of the party.
Its final proposals are due to be unveiled at Scottish Labour's conference in Perth later this month.
Earlier, in Edinburgh, Sir Menzies urged all parties to back his plan for a post-referendum summit.
He said: "It will be important to respond to the desire for change when there is a No vote and to be able to act immediately."
Yes Scotland chief executive Blair Jenkins said: "We have heard warm words before from Westminster politicians about powers for Scotland in return for a No vote. They came to nothing in 1979 and they'd come to nothing again."