Simply get both players' aiming entity and see if they are equal.
PHP Code:
new iEnt[2], iBody
get_user_aiming(id, iEnt[0], iBody)
get_user_aiming(id2, iEnt[1], iBody)
if(iEnt[0] == id2 && iEnt[1] == id)
{
// both players are aiming at each other
}
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