Hammerspoon bind "cmd" & "shift" without any other key

I want to use hammerspoon to replicate Windows' language switching using Alt and Shift. For this I want to add a hotkey bind for command and shift, but all my attempts fail.

I tried:

hs.hotkey.bind({"cmd", "shift"}, "", function()
    performAction()
end)

and

hs.hotkey.bind({"cmd"}, "shift", function()
    performAction()
end)

But nothing works.

philsnow's answer gave me most of the building blocks I needed, here's the code I used to map cmd -> cmd + shift -> shift -> nothing to switch between languages like in Windows. For anyone needing something similar, this code worked for me to switch toggle languages.

function same_keys(t1, t2)
    return _same_keys_oneway(t1, t2) and _same_keys_oneway(t2, t1)
    end
    
    function _same_keys_oneway(t1, t2)
    for k, _ in pairs(t1) do
        local found = false
        for j, _ in pairs(t2) do
            if k == j then found = true end
        end
        if found == false then return false end
    end
    return true
end

nMinus1 = {}
nMinus2 = {}
nMinus3 = {}

function tablelength(T)
    local count = 0
    for _ in pairs(T) do count = count + 1 end
    return count
end

function switchLanguage()
    local lay = hs.keycodes.currentLayout()
    print(lay)
    if lay == "ABC" then
      hs.keycodes.setLayout("Hebrew – PC")
    else
      hs.keycodes.setLayout("ABC")
    end
end

etap = hs.eventtap.new(
   {
      hs.eventtap.event.types.flagsChanged,
   },
   function(ev)
      local flags = ev:getFlags()

      if tablelength(flags) == 0 then
         if (same_keys(nMinus1, {shift = true}) 
                and same_keys(nMinus2, {cmd = true, shift = true})
                and same_keys(nMinus3, {cmd = true})) then
            switchLanguage()
         end
      end
      nMinus3 = nMinus2
      nMinus2 = nMinus1
      nMinus1 = flags
   end
):start()