Hi Tony,
You can get away with one pre skimmer valve if the overall flowrate is strong enough but for the cost of a valve you will be much happier with the tuning ability of a two valve setup.
This is for a single overflow.
The problem is how to get a predictable flowrate to the skimmer without impeding the displays ability to drain (so the return pump doesn't overwhelm the overflow because of restriction). I have put a "step" in the overflow line to force the water flow first to the skimmer and then allowing it to continue on to the sump. You need a valve pre skimmer to control the flow rate into the skimmer. This, combined with the step will provide a constant flowrate in the skimmer but you won't actually block the overflow. You then need a valve post skimmer to fine tune it.
For a two overflow setup, just dedicate one overflow to the skimmer and the second to drain straight to the sump. Set it up so the first drains sooner than the second. That will provide a constant flowrate to the skimmer without risking a flood.
What you should not do is plumb a single overflow through your skimmer only. If the return pump was stronger than the restricted skimmer flowrate the display would back up and flood and.. if the return pump was weaker than the restricted skimmer flowrate you wouldn't be able to tune the skimmer effectively.
Hope this helps