The US1-40 and US11-40 are virtually identical in operation, the US1-40 does not have a faceplate, but uses the single rocker.
The US2-40 is different in operation from the 1-40 and 11-40 and the US22-40 is a two channel version of the 2-40.
The primary difference between the 1-40/11-40 and the 2-40/22-40 is local load operation and status reporting.
On the 1-40/11-40 the rocker can be configured to directly operate the load wired to the switch and report status when this occurs.
On the 2-40/22-40 the rocker transmits a link which is received by the switch to operate the load wired to the switch. Even the load directly wired to the swtich. When you press the rocker it transmits the link and receievs the link and takes action, but does not update its status.
The 2-40/22-40 also use the full range of faceplates.
The 22-40 is a two channel (two load) version of the 2-40. It can operate two loads independently and has two LEDs.
The 1-40/11-40 and the 22-40 also have internal timers that can be set to automatically turn the load(s) off after a preset time.
The 2-40 does not.
To address the individual channels of the 22-40 directly you must use extended unit commands. Something like the HAI Omni Pro can not do this.
It can still communicate via links to each channel, but can't turn "Unit ID 12 Channel 1" on or off directly like it can a single channel unit.
A Western Mountain RUC can send those extended commands.
For instance, I have my porch and driveway carriage lights on a US22-40.
When there is a fire or burglary I turn all the lights on, but I want the Driveway Carriage lamps to blink to alert fire/police.
The 22-40 can not blink a channel in response to a link. IIRC the 11-40/1-40 can (need to check UPStart).
So when the Omni Pro sends the All On command link the RUC also sees this and in turn send a channel command to the 22-40 to blink the driveway lights.
I use that as an example of creatively using the technology and different products to produce the desired end result.
I generally try to connect loads to US11-40s (Or 1-40s) because of the direct operation and status reporting and use 2-40s as link transmitters.
I replace the switches in a three way circuit with one 11-40 and one 2-40 and use the 2-40 to transmit several links for various actions or scenes.