Jim Sillars and Gordon Wilson dismissed the Scottish Government's claim that EU membership would automatically follow independence and claimed "mere assertions" would damage the Nationalists' chances of victory in the referendum.
They called on the Scottish Government to begin negotiations over joining the European Free Trade Association, as an alternative to the EU, to avoid the euro currency.
The EFTA is made up of Iceland, Leichtenstein, Norway and Switzerland, who are outside the EU but enjoy free trade agreements with member countries as part of the wider European Economic Area.
In a statement, Mr Sillars and Mr Wilson said: "The EU may attempt to extract a price from Scotland for continued membership, including joining the euro.
"We would point out to the Yes side, and especially to the Scottish Government, that it would seem wise to have an 'alternative hand' to play in any discussion it may have with the EU.
"As the EU's major oil producer, and important contributor to the Common Fisheries Policy, Scotland is not in the position of a beggar at the Brussels gate.
"But should there be any threat to close the gate, and there have been such threats, then in our opinion membership of the EEA, through EFTA, would seem the wisest course for the Scottish people to take."
The two former SNP leaders also called on the Scottish Government to press the UK Government to take up the offer of official EC advice on an independent Scotland's membership of the EU. They also warned: "Continued access to the large European market, on independence, is one of the core issues that is dominating the debate, and will intensify in terms of importance and influence as an issue that will shape the vote.
"Mere assertions by the Scottish Government that there will be a seamless transition from part of the British state to full member state status, need validation. It also serves the interests of the London Government and No to Scotland to spread confusion: Hence London's refusal to seek a defining legal opinion."
An SNP spokesman said: "Scotland has been part of the EU for 40 years, and an independent Scotland will continue in EU membership as a range of eminent experts have testified."
A spokesman for the First Minister said: "The white paper due to be published next year will set out the Government's prospectus for an independent Scotland."