I would recommend you try the suggestions made in this thread to see what does and doesn't work. Your writing to date implies you have not tried it... you are just asking how it might work. If I've mis-read that... sorry.
To keep one Vive Headset from physically detecting/seeing the "other" two lighthouses laser outputs with its sensors you will have to physically mask between the two sets of lighthouses (maybe hang a thick cloth/material cool banner or a big solid sign ??). So as suggested, hang something of sufficient density to now allow the lasers to pass through it to the "other" headset sensors. Be aware that the laser energy can "splash" off reflective surfaces so aiming of your lighthouses into the specific play areas may play a key roll in your mitigating splash between the two sets in addition to putting up a mask. Steam Workshop has a pretty good PewPew mod/skin for the lighthouses that lets you see the center of aim of each lighthouse and the spread of the lasers to know if you've pointed them well... (when you are runninng SteamVR and wearing the HMD). As I understand it...Vive HMDs don't communicate with the lighthouses per-se... the HMD is used by your computer to register the locations of the light houses, the floor, and the edges of the play area when you run the room setup... because the HMD can detect the light emitted by each lighthouse and reports it to SteamVR software... so the computer knows where the HMD is at any time based upon an effectively run room-scale or standing-sitting setup. Good Luck.