500V/7A is not particulary hard for a switch - or two in series.
I wonder if they need to be mechanically timed (one advances the other) to improve the arc breaking or if slight dimensional differences alone are sufficient. I mean the spark gap doubles over two switches just because it is double the distance, but do resistance differences between arcs inhibit two arcs in series? As in one switch pulls an arc, therefore there is not enough breakdown voltage left for the other switch, or more current = more plasma/ionized gas = better conductivity = lower voltage drop per distance, like a current dependent resistor, should such a concept exist.
This might not be very relevant for a manual operated switch, but for contactors and such.
I think such breakers/isolators exist in in actual high voltage applications.
Although recently i´ve seen a fail with a standing arc for at least two minutes and grass fire from the molten metal.