The biggest advantage you’ll gain by making the deck wider is that you can then build a simple clamshell out of thin plywood and 1×1’s to make the trailer weather tight. With the kart hanging off each side, as it is right now, it would be much more difficult to do this.
Another expansion of the second deck idea would be to build the second deck at exactly the right height that you could slide the kart right off the stand and onto the trailer – possibly even slide it “into” the trailer by making the top shelf an enclosed/weather tight area. This should give you 12-16″ of space between the decks to store your stand and everything that goes with the kart.
Of course at this point you’re basically just re-engineering a 6×12″ enclosed trailer – but I suppose you’ll have much less money invested to get you there.