How would I make a script activated by multiple keydowns?

I believe that you would simply need something to make sure that the user has both keys down before one is lifted. To do this, I would use KeyDown and KeyUp.

Note - Code is not tested.

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
rPressed = false
fPressed = false

mouse.KeyDown:connect(function(key)
    key = key:lower()
    if key == "r" then
        rPressed = true
        if rPressed == true and fPressed == true then
            print("You pressed R and F")
        end
    elseif key == "f" then
        fPressed = true
        if rPressed == true and fPressed == true then
            print("You pressed R and F")
        end
    end
end)

mouse.KeyUp:connect(function(key)
    key = key:lower()
    if key == "r" then
        rPressed = false
    elseif key == "f" then
        fPressed = false
    end
end)