Satellite receivers are full-range, full-function receivers. Their output is a serial data stream over a three-wire bus, instead of a block of plugs for your servos.
Adding a satellite receiver to an existing standard receiver with plugs is the equivalent of having two receivers.
Helis 500 size and up benefit from having at least two receivers -- whether that is two satellites plugged into a flybarless controller or a standard receiver with a single satellite -- makes no difference.
The second receiver adds what is known as antenna diversity or receiver diversity.
RF signals at 2.4 GHz are pretty much line of sight signals. If something that is RF opaque (metal, carbon fiber...) gets between the transmitter and receiver, there is a good chance the signal will either be greatly attenuated at the receiver, or not received at all.
The second receiver, placed in a different location than the first, and with its antenna oriented on a different plane the the first, offers additional RF coverage, improving the overall reliability of the RF link. The system simply uses the output of the receiver with the best reception for its purposes.
Additional satellites only improve the coverage.
The installation you have in your 600 is a typical installation that works quite well.
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