In the Netherlands, the parties PvdA and GroenLinks have started working together very closely, participating in elections with combined lists. They did the same for the EP elections, but had the challenge that PvdA was part of S&D, whereas GroenLinks was part of the Greens.
They went for the same solution: half the list (the PvdA part) will join S&D, and half (the GroenLinks part) will join the Greens. They will align their votes, though.
The way I interpret that, is that it'll be easier to influence the Greens (since it's a smaller group and thus GroenLinks is a larger part of it), but if the S&D position can be swayed, it'll have more effect. And in general, both parties presumably aren't that far from their group's stances anyway.