Also, only run one sensor at a time! Wait some amount of time (say, 30-50 milliseconds) before running the other sensor, and do them ping-pong style, else you may get crosstalk.
You can mount them so they sense in an "X" pattern -- turn the leftmost to face right, and rightmost to face left. This gives you some amount of "center" sensing, hopefully enough to avoid a crash.
Also, try going slowly, not allowing a very fast top speed, until it's all tuned and working well. Then slowly increase max speed.
Finally, to compensate for the shortcomings of US sensors, you can also use a Sharp IR sensor, which works great for soft textiles, but is narrower in its sensing pattern. I use both on my rover robot.