Originally Posted by samundsen
Have you considered using a trailer? That would take the weight off the bikes, and you could choose the bike dependent upon where you're going (dirt roads/gravel/etc, take the mtb, nice paved roads, take the road bike).
I would offer the same thought, except I've been known to blow out rear spokes if they are too light duty. A Burly (two wheeled trailer) would hold it's own weight. Consider gearing AND braking power of either bike.