I'm pretty sure that's it's generally inefficient to purposely try and run two GP farms.
This is true. It is less efficient to run more than one GP farm than to have the same number of total Specialists/Wonders in one city instead of spreading them across 2 cities. In games where the secondary cities earn significantly fewer GP points per turn, it is possible that they will never catch up to the primary GP farm and their GP points will be forever wasted.
It's also less efficient to build military units in cities other than the HE city. I build units in my secondary military production cities too.
The fact that it's less efficient doesn't mean that it isn't effective. Even if I end up devoting 2 extra cities entirely to GP production and I ignore all other aspects of those cities, I still won't be losing money on those cities because the specialists are going to bring in science/hammers/gold/etc and even if I end up with just 2-4 more Great People over the course of the game, it's still going to be worthwhile because I'll get those Great People
sooner than a person who has only one GP farm.
I would certainly get more total

from those cities over the course of the game if I built cottages instead of less efficient specialists, but the fact that I'll get my specialists early will allow me to push forward quickly to a beeline tech and allow my empire as a whole to achieve the broader strategy that I set out toward in the beginning of the game.
I find this much more necessary than in smaller maps because of the added emphasis on bulbs that I have on larger maps. Bulbs between Machinery and Communism/Biology/Steel are much more significant than ones that I get after that critical expansion period and anything I can do to grab those bulbs earlier makes a difference.
Also, when I have fewer cities, devoting 3 cities to Great People generation is much more of a hardship than when my empire is bursting with land.