.......... So I should keep it as light as possible and keep the lenth down but keep the tandem
Yep. I reckon that's the go, Jim.
If you can keep the travelling length to around 5m or less, and match the width and track to your car, I think you should be OK.
Don't go too heavy on the springs. Ours is a roller-rocker set-up, and has 5 leaves per spring pack.
Some other benefits of tandems.....
Less weight on the coupling.
Improved ramp-over and departure angles.
When 1 wheel drops in a hole, the other tends to keep the trailer body higher.
They pivot on the front axle when turning, which helps around tight corners.
Can run 4 wheel brakes if you want.
If things go really pear shaped, you have lots of spare wheels for the car........
Other down sides.......
Extra maintenance.
Can't take tandems into "The Bungles", but that's the only place I'm aware of.
You'll get plenty of experts telling you that tandems are not the way to go.
I haven't found any huge negatives yet.........
Good luck with the build, and keep us all up to date.
Cheers,
Geoff