Hi Ian.
We've caravanned for over 20 years and started with a tiny 10ft sprite and progressed up to a 20ft Link 575 made by Dean line. The van is 25 years old but still in excellent condition.
Towing on the road is much more stable with 4 wheels on the caravan. We notice only minor nodding up and down which is probably due to the linked axle design of our van which self compensates for bumps. Unfortunately modern twin axles don't have the axles linked so consequently you do get a fair bit of weight transfer when going over speed ramps, but having said that you should be doing that at slow speed any way.
Reversing is just the same as a single axle except I try to wake as wider turn as possible to avoid cutting up the grass pitches. Also by not turning in tight radius's you don't put as much side strain on wheel bearings.
As for manual handling, forgrt it. You do need to be reasonable at reversing and also have a motor mover. Again unless you have got an axle mounted mover then you will have to adopt your manouvering to suit. I use a Motor mover Bigfoot on our van which is fine in a straight line but does need a 66 point turn technique to turn the van. However, at half a mile an hour you do have plenty of time to react.
We tow our van with a Renault Laguna 2 Ltr which is front wheel drive, if you have a 4X4 then you might consider fitting a front tow ball to use the car to push the van as you will see better and have more control over it.
Over all though I would not like to go back to a single axle as a twin is more comfortable to tow.
Steve L.