I think one of the main concerns is a blowout. What happens when you lose 1 of only 2 tires? It's going to be worse than losing 1 out of 4.
Also, riding with a tank half full is going to create a lot of slosh. It will have a negative effect on braking, starting, and cornering- in a somewhat unpredictable manner. Like @Innocentbystander stated, ride empty or ride full. Half full is not a good idea.
If you're committed to always emptying the tank before towing the trailer anywhere, then a single axle may work ok for now. But purchasing a second axle should definitely be in your gameplan if you plan on expanding or doing more than part time work.