Another vote for tandem axles. There are various (very good) reasons having to do with GVW, braking, stability, weight distribution, yada yada yada.
The REAL REASON is what happens when you have a flat.
The difference in safety is #1. Typically a flat is a non-event with a tandem...meaning that you might not notice it until you're stopped for gas. You'll have to make a point to be vigilant.
A single with a flat can be very ugly; it will definitely be NOTICED; it will force you to stop and will likely cause you to be working on it on the shoulder, which is incredibly dangerous.
A single might end up *left* on the road while you go for help or a tire.
A tandem, well, you just drive to a convenient place to change it (WalMart parking lot is popular) or the tire store.
Tandem, always a tandem, unless it's just for local use.