Two WDSS sensors will only be approximately in sync, and the sync will vary from trial to trial due to jitter in the computer communications system. This is a matter of OS X and Windows Bluetooth performance. We do not expect two WDSS sensors to be in perfect sync, since they use Bluetooth, and Bluetooth has some inherent inaccuracy in the timing.

In or testing using Logger Pro 3.8, two WDSS units were within 5-8 ms of one another on most runs. Sometimes the sync would be as much as 15 ms off. The 5 ms sync behavior is typical. Our test used a 2.66 GHz Mac Pro running OS X 10.6.2. 

Is this good enough? Well, it depends on the experiment. The response time of the force sensor in the WDSS (or the DFS, as they are the same sensor mechanically) is such that impulses that are briefer than about 25 ms are not displayed accurately. Instead, the sensor displays its own response. This is not a defect, but is the behavior of any mechanical force sensor. If you want to really see the impulse, you need to ensure that the collision lasts substantially longer than the mechanical response time of the sensor. If the impulse is long, then 5 or 8 ms discrepancy will barely be visible. 

If you want to have an elastic collision, you need to use either magnetic bumpers or a hoop spring. Both options give nice long impulses that can be easily integrated in software to get accurate impulse integrals and conserve energy extremely well. These are included in Bumper Launcher Kit (BLK).

In both cases (magnetic bumpers or soft hoop spring) the impulse will last hundreds of milliseconds, allowing the force sensor to respond and show the real impulse, rather than just the instrumental response. 

Related TILs:
How good is the time sync between WDSS and other interfaces?