Load that single axle seriously nose heavy to minimize transitionals, and I think you'll be fine (airbags on the tow wagon might help). Throw more tongue length on it than you need to damp any odd handling that may result from a heavy load, even in balance.

Go dually.
* $66.23 for 4 dually 16x6 6 on 3K-rated 6.5 rims: http://www.lowcosttrailersupplies.com/trailer-parts-oemdwr.html
* $333.95 for single 7K-rated axle with electric brakes: http://www.etrailerpart.com/7ktrailerAxle.htm
Lemme know if you need more help spending your coin. <img src="/forums/images/graemlins/pfft.gif" alt="" /> You'll probably want to call and tell 'em what you are planning to make sure the wheel offset and index diameter works with the drum/hub, but I'm sure you're not the first person who desires the redundancy of duallies without needing the carring capacity and footprint of tandem axles.

Randii