The problem arise when you need to place a decal right over UV seams.
It maybe difficult to implement real-time decal placement over UV seams, as it require a "heavy" image processing.

The workaround by using same 2 decals and matching across the UV, is also not possible. At the current state,
Appearance Editor does not allow more than 2 decimal places for numeric offset values. And that makes it virtually impossible to match anything across the UV.

Minimal 0.01 steps (1 centimeter I suppose) for offset is too high to place decals with precision. I'd prefer 0.0001 step.
Or, if you can find a way to use a single decal over the seams, that would have been ideal.

Screenshot 1 shows my attempt to match decals at the lower back - failed.
Screenshot 2 the easy way using 3DCoat app.