it's very possible to add network support in any emulators , not just n64 (why did you get this idea ?)...
it's "only" a matter of getting player 2 (3,4,...) controller inputs from the network instead of reading the connected controller as usual.
Easier to say it though because, not only the network server/client stuff, it would also need a very well designed implementation in order to reduce latency as much as possible.