I don't know about your S5 but mine had a heat pad covering only about half the build plate. Even if you could put the coils in series, it would be a bad idea. To do things properly, replace the standard heat pad (which is way underpowered to begin with) with a full sized mains heated pad (use thermal fuse as safety).
I have a 1200W unit which is just about right for that size of bed. Control it with an SSR and wire the original heat output to drive the SSR control side.