Tag's pretty much nailed it. It distributes the load better.

The linkage in the middle is called an equalizer. They're not expensive, and not very hard to implement. If you're going through the hassle of building a tandem, there's little reason to do it differently.

Why is it that you think you need tandem?

'06 Dakota QC 4.7 6sp 4x4
'88 Mighty Max
'77 KZ1000 fun old bike