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The dual plug heads were usually made to offer a hot plug and cold plug arrangement for smoother transitions through the throttle range and more consistent runs.
With new carbs and new piston and sleeve designs, those problems are less significant.
Add to that the relatively small market and the high cost to manufacture such a head and you can see why they are not seen much any more.
I, personally still have a YS with Viperhead and an OS 90 with Viperhead that I will fly until they completely die.