stacking concealment

[Mystern]

in the spell description of darkness it states:

This spell causes an object to radiate shadowy illumination out to a 20-foot radius. All creatures in the area gain concealment (20% miss chance). Even creatures that can normally see in such conditions (such as with darkvision or low-light vision) have the miss chance in an area shrouded in magical darkness.

If darkness is cast on a small object that is then placed inside or under a lightproof covering, the spell’s effect is blocked until the covering is removed.

question 1: does the concealment stack?

question 2: does the radius of the effect expand?

in the game on sunday i ruled that the darkness spell is like "anti-light" thus it did indeed stack and 2 darkness spells provided total concealment. but my players argued, saying that it was a spell effect and should not stack unless it specifically says so. if this is the case then 2 everburning torches in the same square provide the same amount of light as 1, right?

 

[werk]

Quoth the SRD:
Multiple concealment conditions do not stack.

1. No
2. No

 

[Infiniti2000]

if this is the case then 2 everburning torches in the same square provide the same amount of light as 1, right?

Not necessarily. Concealment not stacking is an explicit rule. You have no such rule to gainsay the normal laws of physics, so if you don't want your candle luminosity to stack, as DM and creator of the world you have to design that as a physical law.