I don't understand, how are they covering each other if they're overlapping their fire in the center? Aren't they then covering entirely different areas?
with their large arcs, you can create and arc of cover fire that looks like this:
.__ __.
/.........\
The dots are just place fillers to make the figure work.
This is a general look, you can adjust the angles appropriately based on where the force lies. The gist is that you prepare for the army to attack a key route, which should be in the middle of the two mgs.
In the USF case when they approach your set up defense, they can smoke one or both but not before being slowed down by the suppression from being two mgs. simply kite back the one that is smoked not only backwards but towards your other mg's arc. The best way to save an mg is to soft retreat it into another covering arc. Your other supporting infantry should be around laying down fire. Any suppressed targets will remain suppressed so that leaves only a part of the US force free to move in pursuit or preposition.
If you have them covering different areas, as long as they are supported (single mgs die to counter harrasment vehicles or just two units spread out) you can cover a larger part of the map, but the dissadvantage is your forces a spread thin. A long ranged blob, or concentrated mortar fire can open up one of the flanks leaving you vulnerable to a central, and supported flank attack.
Really it boils down to the map, and a judgement call, but given enough practice it will become easier to tell which one of the set ups will work best at the give time.