Who says? The media. I don't belive that the PC and CA have to unite for the Liberals to be defeated, it would help a great deal, but it is not required. Nor is such a merger anywhere close to being likely. It's a red herring that is perfect for discouraging conservatives and fostering a defeatist attitude once the election is called and there is no longer any time for such a merger. Already there is a great deal of defeatist attitude out there.
There is enough room among the 10 million non-voters for BOTH the CA and PC's to do better than the Liberals in the next election, and I don't think it's realistic to assume the Liberals will still get the 6 million votes they got last time (and I hope to make sure of that).