Originally Posted by BlueFlash
Another thing to consider is the load placed on a double axle when turning tightly. Even with the wheels as close together as possible they will twist and that load (at 90° to the wieght load) can exceed the capacity of the bearings and lead to failures. So Replacing a single 1500Kg axle with 2 x 750Kg or even 2 x 1000Kg axles is not advisable. I would go for 2 x 1500Kg to replace a single 1500Kg. Sounds like overkill but its really just understanding the immense strain caused by that turning twist.
The only problem with replacing with twin 1500kg axles is that you'll have a very harsh ride.
I would go with a replacement 1500kg or upgrade to a single 1800kg one. Keep a spare hub and bearings and on long runs on the road throw the spares in the boot so that road side repair is possible to get you home should a wheel bearing fail.