[zarlor]

SWD p. 48 "A character who wears multiple layers of armor only gains the highest bonus—they do not stack" That rule still applies to "natural armor" as well. So that's the default behavior for armor. If your Force Field belt is from the Force Field power, since that power doesn't override the armor rule, it wouldn't apply. If it is actual gear from a setting, however, that could override that rule.

Still, the PEG forums are still down so I can't check for certain if Clint may have ruled specifically regarding force fields potentially stacking. But even if you could add a piece of armor that was supposed to be stackable (let alone a power) you'd have to set it up so that it wasn't an armor bonus, it would have to just modify Toughness directly but NOT as an armor bonus. Most likely your Force Field Belt is set up as an Armor Bonus so if it's supposed to be stackable you'd have to create a different version that just directly modified Toughness.