You not familiar with the Alpha Q Z-Pro fork which is specifically designed for the way I'm using it with no spacer limit. The steer tube on Z-Pros is at least twice as thick if not thicker than a normal tandem fork and maintains that thickness throughout.
It's the same fork Zinn uses all his custom built bikes for really big guys who need extended steer tubes and/or stronger all carbon forks.
Check the AlphaQ website for futher info.
KRhea