You need only 4 wires to connect all input multiplexers together using 4 common address lines: S0,S1,S2,S3 (or pins #22,23,24,25) and, of course you need to use 2 power lines - +5V and common Ground (GND) for all boards.
The SIG (can be labeled as "Z" on some boards) output of every input multiplexer should be connected directly to the pin assigned in the configurator for this input extension.
Note: The "EN" signal of each input extension board should be always logical "0" (set to GND). Usualy the multiplexer breakout board has a pull-down resistor on this 'EN" pin, so it can be already set to logical "0" but you need to be sure it is, so just connect it to GND.
The RealSimControl firmware code algorithm guarantees fast and lossless readout of all input states, no matter how many inputs are assigned, and sends data packet to the plugin, which synchronizes data receiving/sending.
The signal "SIG" (can be labeled as "Z") pad of every input multiplexer should be connected directly to the pin that need to be expanded. The most reliable way of wiring these extension modules is to place the board close to a group of switches and solder them with thin wires:
If you noticed that some of your assigned switches or encoders are not stable, for example they send excessive On/Off signals when you activate them, do not cosider this a switch bouncing problem, as RealSimControl firmware code has good debounce protection. If you see that switches in your virtual plane cockpit are occasionally "flicking" when the real swithes are in OFF state, that means you have either missed common GND or "noisy" electric environment in combination with rather long wires.
The multiplexer Z (or SIG) output wired to the master board input has high-impedance "open" state and the wire can catch such EM interference that depends on the electic noise level in power source lines and other surrounding wires. Although every assigned controller board input pins already have internal pull-up resistors connected, their 50k nominal could be not effective enough for long wires.
Thus, if you have long wires between the switches and the multiplexer inputs, or between the master board and the multiplexer "Z (SIG)" output, you need to place an additional pull-up resistor ( 2 to 10 KOhm ) between MUX Z (SIG) output and +5V as shown in the picture above.
Place the master board somewhere in the "middle" of your whole cockpit, in this case the length of all address wires will be no more than 1-2 meters. Also, I advise (as I always do) not to use any "extra" boards (PCBs), other than ready-to-use MUX boards, and to use soldering as I described above.
But if you already have such "additional" PCBs, do not use too many "detachable" connections/headers, especially if you have many of those boards that are daisy-chained (with go-through "in" and "out" connectors). This is very unreliable and can lead to multiple contact loss, can add a lot of resistance to all bus lines (due to oxidation of contacts), etc.
I recommend using the multiplexer boards as they are, fix each board next to the groups of switches and solder them with a bundle of 4 thin single-core wires (very clear wiring).
You can add multiple simple contact pads at different locations to branch the wires (you can solder another group of 4 wires to these pads in different directions, as I show in the right picture here as an example).
Instead of these contact pads, of course, you can use some kind of wire splitters with reliable wire fastening, like these ones:Example link 1 Example link 2 Example link 3 (note, they are for the minimum wire size of 28AWG..)
You can group a set of switches into one module/panel with multiplexer extension boards. Place several modules in different parts of your cockpit and connect them to the main board using only one pin for each multiplexer and the common 4 address wires that are laid across the cockpit to each multiplexer: