table1 = {}
table2 = {}

behavior = {myfunction = function() return 10 end}

setmetatable(table1, behavior)
setmetatable(table2, behavior)

a.myfunction()   --> 10
b.myfunction()   --> 10

behavior_derived = {myNewFunction = function() return 20 end}

-- merge the base behavior in with our derived behavior
for k,v in pairs(behavior) do behavior_derived[k] = v end

a_derived = {}
b_derived = {}

setmetatable(a_derived, behavior_derived)
setmetatable(b_derived, behavior_derived)

a_derived.myfunction()      --> 10
a_derived.myNewFunction()   --> 20

b_derived.myfunction()      --> 10
b_derived.myNewFunction()   --> 20