An electoral pact to achieve PR?

It is beginning to look inevitable that the Labour Party will split - at least in the sense that the majority of the PLP will separate themselves from the mainstream of the Party. Assuming that Jeremy Corbyn is re-elected, this could happen either by Labour MPs resigning the Labour whip, or having the whip withdrawn (but who will control the whip?), or by just being deselected. In either case they could not then stand as Labour candidates in the next election. They could then either start a new party (SDP2), or join the LibDems.

If this happens then the Conservatives will undoubtedly win the next election (presumably in 2020) - unless.......