Of course you could have that one then 'tree out' into additional pipes ( every Nth msg goes to one of several load leveling network processors each with its own outboud pipe)
But I was thinking of various keepalives, sorting, message grouping (fewer teenie packets) and subsetting operations the network node might be able to offload.
Some of the MMORPG Ive seen recently start to choke because they arent just sending position/action update info, but also more dynamic specific object info as the player (or various objects) moves. Future game will probably have an explosion of the amount of data sent, but as you mention 'big' games already are clusters of machines (and have been since the first mmorpgs -- wasnt it Everquest at one tiem that had hundreds(1000s?) of off-the-shelf PCs running their client layers??)
As for the original single send question, you can 'send' via a single socket internal shunt from one core's process to another and THAT process can do the multiple sends (subscription list out to clients).