I understand the idea of a remain alliance is appealing, but I do wonder if it would work as well as people think. A lot of parties are kind of dependent on not being seen to be on the same wavelength as the other parties who have an ostensibly similar view on Brexit.
The Lib Dems can't stand down for Labour because they are absolutely dependent on winning over right-wing remainers who don't like Brexit but REALLY don't like Corbyn. They will lose a lot of votes (let alone a few MPs) if they are seen to be working with Labour.
And although Labour want a second referendum, they aren't positioning themselves as remainers. They have a broad base which does include people who voted to leave. So the hardcore Brexit nutters aren't going to vote Labour at this point, but if Labour stood down for the Lib Dems whose position is to revoke Article 50 entirely and never speak of it again? Not necessarily going to help with more moderate Brexit voters.
Labour and the Lib Dems are surely pretty dependent on unionist votes in the very few places in Scotland they have a shot, so standing down for the SNP in most of Scotland would probably lose them votes elsewhere in Scotland, and certainly could lose them votes in England.
On the other hand, I think everyone could stand down for Caroline Lucas in Brighton, and probably should because saving polar bears is just about the only thing all voters left of Farage can agree on.