We have them in helis though depending on the engine they are more a nusance than a help. On the 91's though, the low and mid range noticably smooths out when the second plug lites off.

They are dual plug heads called Viper heads.

Though most of us run 30 percent every weekend with single plug heads just fine. If its acting up on high nitro the best thing to do is play with shims and lower the compression ratio.