L5 and L6 are partially saturated in normal operation, by the forward DC current that passes through them via both legs of D23.
Sending current through them in the other direction via D30 and D31 reduces this net DC component through both coils, which increases their inductance and therefore their impedance, reducing the output voltage.
It's really a kind of magnetic amplifier.
G36 found this paper that explains the application in detail: "Magnetic Amplifier Control for Simple, Low-Cost Secondary Regulation"
Two fuses, for two reasons.
First, I want to establish some arbitrary values. For the purposes of the below, I will assume the devices will draw 1 amp while idle, be connected to either one 10 amp fuse, or two 5 amp fuses, and become damaged at 7 amps.
Depending on how sensitive these devices are, limiting the current draw of the two devices may not be sufficient to prevent one from becoming damaged. Imagine that one or both devices are idle when one device suddenly malfunctions. A short circuit causes it to draw 8 amps of current, well beyond the 7 amp damage-point. With a single fuse, the total amp draw is 8 + 1 = 9 amps, so the fuse doesn't blow and the device is damaged. With two fuses, the five amp fuse on the malfunctioning device blows, preventing further damage. This isn't a contrived scenario, most devices are in standby most of the time, and, although some malfunctions can cause sudden power spikes, others can cause slow burns. I could answer the question more fully if I actually knew what the devices were.
Another reason is that, depending on the nature of the devices (From your question, maybe high power electronics?), I could imagine that one may internally malfunction and destroy the other. I really couldn't say whether this was a possibility without more information, but I would consider it. With a fuse on each device, each device has its own protection and you don't have to worry about hooking up another device to the circuit, removing one device from the circuit, or changing the nature of one device. You can rest assured that each is separately protected from a surge. It's just good practice.
If this device is a low-cost device, a low power device, or a temporary situation, it wouldn't be worth the bother. I would say that if it was worth the effort of asking the question, its worth a couple of bucks for a separate fuse.
Best Answer
The majority of devices found in the home or office are wired this way and there are few issues. The problem is that if you live in a land of non-polarised mains plugs which when reversed, can make the non-switched wire live, then the internal circuitry remains live even when the switch is off. A table lamp, for example, has a single-pole swtich and the terminals are exposed when the lamp is removed. Your setup should be much safer if boxed up properly.
When the switch is off both lines are cut and the circuit is completely dead.
Check your SMPS and you should find that there is a fuse on the mains side of each. The main fuse protects the wiring inside your unit in the event of a fault. Your SMPS units will supply a total of about 100 W so will draw a max of 130 to 150 W fully loaded. In 110 V land this will be about 1.5 A and in 230 V land it will be less than 1 A. Use a 2 A or 1 A fuse as appropriate in the common live line, point 'A' as you have correctly shown. This will limit any fault current to a much lower and safer value than your mains wiring can provide.