But when multiple emitters are emitting I'd want any receivers present to see a carrier signal from them, and not to get destructive interference in which one emitting unit's LED is in the ON part of its carier wave and another unit's is OFF at the same instant, which then swaps over so that two signals 180 degrees out of phase are added to give a long constant ON period and "destructive interference"*.
In networking protocol language vocabulary, what you appear to be describing is indeed an optical version of a packet collision. It's an inevitability with any asynchronous wireless communication, whether it's HF packet radio, 2.4GHz WiFi or LED.
Is synchronising signals for such a situation possible? At these sort of carrier frequencies? What sort of topics and component types does one look up when considering this?
CSMA/CD (or in the case of wireless networks CSMA/CA) can be implemented in your optical systems, but if you don't want to deal with that, an alternative technique (for low bandwidth data) is to use GPS synchronised time division multiplexing (TDM) i.e. device 1 transmits on the 1st second of every minute, device 2 transmits on the 2nd second of every minute, device 3 transmits on the 3rd second of every minute, etc, etc. As long as each device has a dedicated time slot, and each device knows the other devices' time slots, then you won't get any packet collision.
You could potentially use a second LED master clock channel to substitute for the GPS (and a master clock generator), but this will likely interfere with your existing LED data channel (unless you can somehow arrange for a "green" LED to be received only by a "green" receiver, and "red" LED to be received by a "red" receiver, in order to separate the clock and data channels*). Another substitute for GPS for a master clock would be a clock pulse transmitted over 433MHz ISM.
Note that TDM won't work with independent clocks inside multiple transmitters, because the clocks will drift relative to each other, causing the data to go out of sync.
*"separate "red" and "green" LEDs are effectively a form of frequency division multiplexing, though normal FDM would be data in both channels