The thing that annoyed me for some time was that my colleagues, using gnome, could resize their clients/windows to span both screens on our dual screen setup at work. So after some thinking and help from the awesome channel I wrote a lua function to do that with a client.

The function

 function fullscreens(c)
     awful.client.floating.toggle(c)
     if awful.client.floating.get(c) then
         local clientX = screen[1].workarea.x
         local clientY = screen[1].workarea.y
         local clientWidth = 0
         -- look at http://www.rpm.org/api/4.4.2.2/llimits_8h-source.html
         local clientHeight = 2147483640
         for s = 1, screen.count() do
             clientHeight = math.min(clientHeight, screen[s].workarea.height)
             clientWidth = clientWidth + screen[s].workarea.width
         end
         local t = c:geometry({x = clientX, y = clientY, width = clientWidth, height = clientHeight})
     else
         --apply the rules to this client so he can return to the right tag if there is a rule for that.
         awful.rules.apply(c)
     end
     -- focus our client
     client.focus = c
 end

This function will:

• Toggle the floating property on the provided client as parameter.
• If the floating property becomes true it will resize the client on all screens.
• If the floating property becomes false it will restore the client's default size and tag.

Some notes on the function

On some machines the most left screen(from where the geometry coordinates should start is not screen[1] so

 local clientX = screen[1].workarea.x
 local clientY = screen[1].workarea.y

should be changed to

 local clientX = screen[other_index].workarea.x
 local clientY = screen[other_index].workarea.y

Function usage

In my configuration Mod + f is bound to fullscreen, so Mod + Shift + f seems like a nice key combination for fullscreens. To use that put this line where you define your clientkeys

 clientkeys = awful.util.table.join(
     ...
     awful.key({ modkey, "Shift" }, "f",        fullscreens),
     ...
 )