Once the front fork is secure, you might be surprised at how stable it will be. That's not to say at a minimum, four straps are required, and more is better too. Like Yota does, four straps on the front is a good layer of redundancy. Straps are cheap - dented fuel tanks aren't. In a two straps on the front config, if one of two straps lets go, say, one of those fine and dandy Chinese strap buckles pops open or fails, it's all over but the cryin'.
Be aware that the top of the rear shocks is a tricky place to strap. I had one slide off there due to rough roads.
