To my mind, the most likely outcome – and one mooted in the papers today – is a three-way split. That is: Lib Dem ministers voting for the government's policy; most Lib Dem backbenchers abstaining as per the coalition agreement; and a handful of disgruntled Charles Kennedy types voting against it. From the Lib Dems' perspective, it is not an ideal outcome, and enables their opponents to mock them as internally riven. But, then, there is little about this situation that is ideal for the Lib Dems, and other outcomes could be more damaging in the long-run. If, for instance, Lib Dem ministers chose to abstain, then they would dilute their party's claim to be a credible member of government.
Coalition does rather entail messy, uncertain situations such as this. But Cable may still look back on the past few months, and wish he'd kept some of his deliberations behind locked doors. From proposing a graduate tax to backing down from it, from abstaining on tuition fees to voting for them, the Business Secretary has hardly been a model of consistency.