The conceit is that if fiscal cliff talks fall apart, Republicans, by passing Plan B, will have scrambled the CW that they're only interested in protecting millions and billionaires. That'll hand at least partial ownership of going over the cliff to Democrats -- and thus, perhaps, convince Obama to concede a little more.
The risk, as I see it, is that if Plan B passes (still a big if) it'll be hard for Boehner to return to the negotiating table at all. His conference will have just overwhelmingly thrown its weight behind a much tamer plan, and in the process Republicans will have taken a huge leap of faith, 20 years in the making -- knowingly voting to raise taxes.
How can he go back to them a few days later and say, "Just kidding guys, that was a practice run. The real deal has way higher taxes!" I don't see it going over very well.
Of course, if Plan B fails on the floor, Boehner's going to have to choose between the cliff and Obama's outstretched hand. And perhaps for that reason, deep down, he wants it to fail. But there are some restive conservatives in his conference who will be supporting Plan B, as the lowest-cost way of getting the tax issue off the table altogether, so they can pivot as quickly as possible to the debt limit.
It would be a huge irony, but if that bloc of Republicans casts the decisive votes for Plan B, tax increase and all, we'll be in new, uncertain territory.