Here's how one guy did it;
custom made shackles to tie the springs together.
And here's how I did it;
Uses all stock Tamiya suspension parts combining 2 rear suspension assemblies
Look closely at the center axle;
that's a radius arm bracket on top - one spring eye bolts into it, the other spring eye slips along an aluminum piece glued inside the bracket on top of the bolt heads. You can't bolt both spring eyes into the bracket, then there is no articulation.
Here they are set up on my B-trains;
