While it is not difficult to re-space the frame to accommodate a wider hub, 8-sprocket freewheels are more prone to axle breakage than 6-sprocket freewheels, and on a tandem this could be a significant concern.
If it were my bike, I'd either replace the wheel with a freehub wheel to provide better axle support for an 8-sprocket cluster, or just live with the 6-sprocket wheel.