I finished two N64 portables and built a LOB64 cable and tried this out. I found out that it only works if both N64's are on.
That means no switch to turn the N64 off. The only thing needed now is to wire a port, and that's LOB64.
It's too bad it won't work with the slave N64 off, it saves some power.
How you determine which portable is master or slave now is by which one has a cartridge in it and by the cable.One side of the cable plugs into the master, and the other into the slave. It has to be labeled, plugging the cable in backwards won't work. (See diagram in first post)
Without both portables (controllers) on, the master works fine, but when you plug the cable in to both portables it makes the master freeze until you take it out again. If both portables (controllers) are on, the master does not freeze, but the second-player controller acts erratic, pressing tons of buttons in a row. If both portables have their N64's on, everything works fine.
There must be some kind of interference loop with both N64's being grounded together. In any case, it cannot be fixed, so the only solution is to have both N64's on.
Sorry for any inconvenience!
I will put up a video soon of my N64 portables and LOB64 in action.