For mitt Romney to agree to join the Democratic ticket would require that the GOP candidate is far too extreme for Romney and that his joining of Cuomo's ticket would be supported by other establishment figures like John McCain, the Bushes, etc. Cuomo/Romney are able to win by attracting the votes of traditional liberals (think John Kerry) and many traditional conservatives (think John McCain). The GOP is able to win only in the most deeply conservative parts of the country. Meanwhile the independent ticket is able to pick up wins in a few of the most progressive states.
Democratic Ticket: Cuomo/Romney
GOP Ticket: Ted Cruz / Rand Paul
Independent Ticket: Bernie Sanders / Keith Ellison