Have you played with the following, taken from the wiki?
...
local SS_SID = "urn:micasaverde-com:serviceId:SecuritySensor1" -- Security Sensor Service ID
local armed = luup.variable_get (SS_SID, "Armed", deviceNo)
if (armed == "1") then
...
Have you attempted something along the following untested lines?
local SS_SID = "urn:micasaverde-com:serviceId:SecuritySensor1"
local armedState1 = luup.variable_get (SS_SID, "Armed", <dev1>)
local armedState2 = luup.variable_get (SS_SID, "Armed", <dev2>)
if (armedState1 == "0") and (armedState2 == "0") then
return false
else
return true
end